With my vote for retail, the votes are split evenly, which I feel is appropriate. I feel like screenshots on covers meant to act as retail should be recreations as close as possible to the source material. The screenshots on my GB/GBC/GBA Mega Man covers are all very, very close to being 100% accurate to their retail versions, although I initially captured most of them with emulators. I used MS Paint afterwards to rearrange elements of the screenshots to more closely match the boxes where needed. The screenshots on my MMBN 1 & 2 covers were completely rebuilt from scratch, however, just since there was so much going on in those shots that it was nearly impossible to recreate on the fly (plus one of MMBN1's screenshots required access to a prototype version of the game that I obviously don't have, so I HAD to rebuild it).
Yes, perfectly matching the screenshots (or even coming incredibly close) is time-consuming, but I feel like paying even that amount of attention to detail only serves to better the cover as a whole.