1) Not sure, but most 20k word bangs ask for 1 (or sometimes 2) really good drawn pieces from the artists, or one vid/fanmix.

2) trekreversebang is the first bang I know of that's flipping it - the artists create first, the writers pick art to write from as a visual prompt. They acknowledged that it's harder to write a story based on a picture than it is to illustrate a written fic, so the word count for the writers was dropped to only 6k, instead of the 15-50k normal for big bangs.