Over at Worldbuilding we constantly bring images and quotes in from outside sources. I suspect other Stacks do that a lot, too. When it comes to images, charts, and graphs, we want people to properly attribute the graphic. It's not just a courtesy to the creators of the graphic (who certainly deserve every ounce of credit), it's also an effort to protect their legal copyright as the graphics may not be sourced under the same copyright conditions that Stack Exchange uses.
Request: What I'd like to request is a means of specifically identifying a block of text as an attribution. I don't know that it needs to go so far as to be programmatically attached to the graphic, although that would be nice. I imagine something like the single-asterisk list block or the close-angle-bracket quotation block. Not only would this provide uniform formatting for attributions, but it would also allow text readers to properly identify the purpose of the block and, hopefully, would encourage more people to include attributions.