The answers are here!

Ghouls

Creeper
http://www.halloween-masks.com/nosferatu_gothaw.htm
Part 1: His head is taken from this mask. The mask was cut in half and mirrored just like Frostbite. He obviously has had some further photoshopping done to him, the most obvious being blackening his eyes.
Part 2: His body is a zombie body from an unknown source. I cannot find this at the time.

The Yurei:
http://www.youtube.com/watch?v=qttT_10QGQ0
This one was easy. Just ripped from this video and slightly edited.

Eyesore
http://halloween-mask.com/dr_cyber.htm
With a little MS Paint legs, you have him.

Jitterskull
http://halloween-mask.com/adults_skull_ru66020ih.htm
Blacken the eyes, add small red ones, and their you go.

Frostbite
http://halloween-mask.com/crypt_creature_fw8517_morris.htm
This ones a bit tricky to see. Because you can only see him from an angle, you have do a bit of photoshopping. You have cut off the left half of the picture and mirror the right. Get rid of everything but the head and finally remove some of the hood and whats left is frostbites, hood/hair.

Choke
Choke is made from a painted clay or ceramic mask.
A link is currently unavailable as the root image is unavailable.

Sjas
SKIN: Sjas is an image (most likely) from a website http://Bloodlustsoftware.com. It was a skewed pic that was later un-skewed. A link is currently unavailable as the root image is unavailable.
WEAPON: Sjas's scream is taken from a popular screamer called "Whats Wrong With This Picture?" http://www.moillusions.com/2006/03/scariest-optical-illusion.html WARNING: DO NOT CLICK YOU HAVE HEART PROBLEMS OR ISSUES WITH SCARY IMAGES

Humans

Marine
http://doom.wikia.com/wiki/Doomguy
SKIN: If you are seriously asking this question, you deserve to be shot. The only new thing is his standing still animation.
WEAPONS: The machine gun is from the famous Alien's TC.
His hand grenade is almost definitely from here:
http://www.realm667.com/index.php?option=com_content&task=blogsection&id=30&Itemid=157&limit=20&limitstart=20
His riot gun is possibly from a mod that Wild Weasel made.
CLICK THE LINK AND HIT "CTRL+F" (ON PC) or "COMMAND+F" (ON MAC) AND SEARCH "ReDoom"
http://forum.drdteam.org/viewtopic.php?f=5&t=20

Hunter
http://doom.wikia.com/wiki/Cleric
Just as Cutmanmike said in his description of him: Simply, Parias the Cleric from HeXen:Beyond Heretic. He has been edited to have a bow as opposed to a mace.

Ghostbuster
http://doomworld.com/gbd2/main.htm
SKIN:Ghostbuster is taken from the uncompleted Ghostbusters TC. More specifically, the Peter Venkmen skin. An interesting note is gibbing (EG. Dieing from a Jitter, Frosty, or Choke) your head that bounces, is actually the Ray Stantz skin's head at an angel. This is noticeable because you can see a bit of his cigarette and you can also see he is frowning which Venkmen doesn't do, Ray does.
WEAPONS:His traps themselves are again taken from the unused GB TC. However, his proton cannon was redrawn for this project by Steve "Scuba" Browning. All of his upgrades are new sprites with one exception: the blood spreader. The very tip of the blood spreader is taken from the tip of the "Slime Cannon" from the GB TC.
FUN FACT!: A group of people are bringing back the Ghostbusters TC!
I (Geonightman) am the lead mapper of said project, if you have any questions, come to the forum or ask me.
http://www.doomworld.com/vb/wads-mods/50069-ghostbusters-doom-looking-for-mappers-and-decorate-programmers/
You can follow the link if your interested or if you want to download the skins, go to page 4

Engineer
http://www.doomworld.com/idgames/index.php?id=10706
SKIN: Taken from the "Base" skin of the Gothic99 deathmatch WAD.
WEAPONS: The nail's shot out by the nail gun are from various wads that have a nail gun in it of some kind, like NEO DOOM and Doom Raider. The actual nail gun is a gun called to ENFORCER. The enforcer can be found at:
http://www.realm667.com/index.php?option=com_content&task=blogsection&id=30&Itemid=157&limit=20&limitstart=20
The Saw Thrower is a popular weapon in wads. It can be found here (most likely):
http://www.realm667.com/index.php?option=com_content&task=blogsection&id=30&Itemid=157&limit=20&limitstart=40
As for the Steam Cannon, it was taken from a weapon called the Coal Cannon, found here:
http://realm667.com/board/viewtopic.php?f=4&t=189&start=0

Cyborg
http://doom.wikia.com/wiki/Reaver
SKIN: The Reaver from Strife. Nuff said.
WEAPON: His gun is from the Samus Weapons wad and the fire sound is the Arachnotron's firing sound from Doom for the Playstation. (These sounds were RE-USED for Doom 64, so technically they are NOT from Doom 64, but Doom for the Playstation.)
If you are set to female and you taunt OR you use the female bot, you will here that its voice is taken from VALVe Software's "Portal". More specifically, the turrets.
Get the Samus weapons here:
http://www.doomworld.com/gbd2/weapons/weapons/samus.htm

Santa
http://www.doomworld.com/idgames/index.php?id=4904
SPRITE: Taken from the "Base" skin of the Head-2-Head X-Mas WAD.

The Creepers body, Sjas's sprite, and Choke are still being looked for.

ALL STUFF FOUND BY GEONIGHTMAN! ANYONE ELSE WILL BE CREDITED HERE:
  SPECIAL THANKS TO
Cutmanmike for creating Ghouls vs. Humans to begin with.
Cutmanmike (again) for helping me find the Creeper's face.
Captain Ventris for correcting my typo about the Cyborg's sprite and telling me about the Hunter being slightly edited.
Ivory for helping me find the Steam Cannon's origin.

.

I have a question for the lead mapper of the Ghostbuster TC. When do you think you'll be adding in Raymond, Egon, and Rick Moranis?

It appears Sjas has been lost in time. I checked the whole website ( http://bloodlustsoftware.com/ ) for him. I could not find Sjas's picture, skewed or otherwise.

I have not given up on choke, but Google Images has yielded nothing from the following searches (These are based on both this forum what CMM said and what CMM put on Chokes profile page: Clay skull, red clay skull, 3 red clay skulls, 3 skulls, 3 clay skulls or 3 ceramic skulls, ceramic skull, red ceramic skull, 3 red ceramic skulls, 3 ceramic skulls. Wasting a good 2 hours looking.

As for the creepers body, I can't seem to find that on Google Images anywhere. I only searched one word though: Zombie.

Mike, if you can remember where you found these at all (except Sjas which we already talked about) it would be greatly appreciated. It doesn't even have to be the exact site, just what you used to find it, like which search engine, or what in particlar you searched.

REQUESTS: I was wondering, you keep talking about this wad you and your friend try to scare each other with from time to time that you take turns updating. Could I get a copy of this wad OR you possibly put it on the site.

Can I answer this!?

Now if I was cutman, I would totally be like, "but most of what you found I discussed, and everyone kind of already knew this."

:x

@Cutman
Do you happen to remember where you got the engie's weapon sprites?

I believe they're all on Realm667's weapons archive, although I didn't take them from there except for the enforcer.

Quote from: "Mobius"

Can I answer this!?

Now if I was cutman, I would totally be like, "but most of what you found I discussed, and everyone kind of already knew this."

:x

While I admit this is true. I did put some effort into this. It was rather difficult to find some of these. And before you say I didn't find all of these, if you read the bottom of the post, I do give thanks to those who help me find it.

With all of those factors, you didn't really find anything :x

@Cutmanmike
Since I'm part of reviving Ghostbuster's Doom I would notice this: Why do you play as Peter Venkmin in GvH but when you are gibbed (EG. Eaten by Jitter, Choke, or Frosty) your head is Ray Stantz?