Wow. Very cool.
As soon as I saw this community, I wanted to jump in and become involved. Having skimmed back over what people were offering, here's what I'm willing to donate, and, well, what I'm best at:

Photoshop work. Specifically, I'm best at turning people into Star Trek aliens and/or White Wolf-style Changelings, but I'm willing to give a go at whatever, as long as you can provide me with -- or point me toward -- a few clear, decently-sized source images and good descriptions of what it is, exactly, that you want.

Simple manip: putting a head on a uniform/changing clothes/simple background (up to 640x480): $2.
Desktops: See above, but up to 1024x768: $3
Alien/changeling manips, simple: Turn Amber Benson into an Andorian? Mischa Barton into a Bajoran? $3.
Alien/changeling manips, complex: Turn Christopher Walken into a Cardassian? $5
Multiple individuals in an image or complicated backgrounds: Add $1 per individual, or $2 per complex transformation (i.e. 2 Cardassians).

These images generally take me about 24-48 hours to complete, work schedule permitting.