It's been ages since I've seen a reverse big bang -- I used to be involved in a fandom (not mentioning it out of SHAME) that did quite a few of them. One interesting thing they did there to make up for the fact that most fandoms have more writers than artists is take a panel from an older not iconic issue (i.e. in this case nothing from "Civil War: Confessions" or "Demon in a Bottle" )of the series, remove the text and submit it as a prompt. Interesting picture, no context. Or derivative covers could work too.

If you needed to cite the source you could just go by volume/issue number, artist -- if the writer wants context (which ruins the fun imo) they can find it for themselves.

The "shippy moments in canon" shots might be good source to draw from too.