you could also create a signature by doing the following:
1. right click on a picture you want by selecting "save as"
2. click "okay" button
3. go to your user cp on ad and select "edit signature"
4. select "browse" to find a picture you'd like to have appear in your signature.
5. highlight the picture and select "okay"
6. click "save signature" or "preview signature" if you'd like to see your signature before saving it
You could just make a "Signature Image Request" thread so that it can be for anyone.
What if one member wants to request a signature image, having a neutral title like that would allow 3 or 4 members to create images and post in that thread... then that one member can choose from those images.