I took part in a forum game like this on another website a while back and remember having loads of fun with it. I also stole the title, would you believe?
I'm sure you've seen it before. Basically, the idea is that someone posts a source photo and everyone else manipulates it to their will (using photo-manipulation software like Photoshop, Gimp, or MSPaint if absolutely necessary). After an amount of time, the person who posted the original photo selects a winner, and the winner posts the next source photo and it just goes on and on like that.
The last site I did this on was considerably more active, so they gave a day between source images. I s'pose we could start this off with a three-day time limit from posting of source image to declaration of winner and see how it goes from there? It's based on the activity of the members so there's going to be some leeway regardless, but three days seems like a good place to start as a guideline. (EDIT: 5-7 DAYS SEEMS LIKE A GOOD PLACE TO START AS A GUIDELINE)
Example:
The winner posts a good, high quality source image.
And others have a go at photoshopping it.
Then, after 5-7 days, the source-poster selects a winner and the winner posts another source image, and so on.
So, if you want to take part, just take part! Obviously this works a little better when more than one person is actually doing stuff, so I encourage you all to give it a red-hot go. It's good fun and great for practice with Photoshop as well.
And, here is first source image:
[The high-res image is in the attached thumbnails area. I honestly have no idea how to get those actual, full-size images on the post without stretching the page, so I just uploaded them twice in two different sizes]
Let me know if I missed anything, and enjoy!