We have to remember that using Photoshop and doing art are two totally different things. Even though you had used Photoshop for 10 years, that doesn't necessarily mean you can draw art, and it seems to me that this project is more about doing art than learning to use Photoshop.
Then again learning the basics of Photoshop is of course where we should start, and while we are waiting for vrt to make a glorious return, we could start this project with Photoshop basics.
Just have one volunteer to become a Photoshop "teacher" who starts maybe 6 lessons (topics), one by one. Each lesson would have one main topic like for example layers. At the end of the lesson, students are required to post their "homework".
Short version: Teachers starts a new post with a simple Photoshop tutorial. Students read the tutorial, produce that "homework" image, and post that image on the topic.
Then when it comes time to start the art thing, there's already some Photoshop knowledge among the students, which enables them to move directly to drawing stuff.
Example lesson plan (made in 3 minutes):
LESSON 1: New Image + Saving an Image
LESSON 2: Painting Tools
LESSON 3: Other Tools
LESSON 4: Adding Layers
LESSON 5: Image Adjustments
LESSON 6: Adding Effects
Something like that. Won't make you Photoshop wizard, but would give you a good solid foundation relatively fast.
It would be cool to have a big Photoshop Class and Art Class, and a person that finished both, "graduates" and gets a forum award icon.