What if we try to build a repository of prompts? Topics/themes to inspire people and give them concrete ideas to work with. For example, I think it might be cool to have the creator of the weekly featured image do a short post explaining what inspired the shot and/or how s/he achieved the effect in the shot. It's easy, it's relevant to the site, and there will always be material for it. (This would, of course, require that the weekly featured image winners be willing to do that, but I think it would be a neat series. Even as an occasional thing - once every three or four weeks, since I'm sure not everyone will want to write.)
And just to keep in mind: blog posts don't have to be long. A couple paragraphs is plenty. It doesn't have to be a burdensome task that will take weeks to complete. :)