Not sure it could be limited to that but, i've looked into something similar before where users can add a background behind their post, that image could be made transparent I suppose to be more of a watermark. The issue with it is that not everyone has fast internet and is using their mobile devices to be on AB. The other issue is that the site as a whole could look like crap when people start using low quality images. Anyway I can look into it further if it's something more people would like to have.