User blog comment:PlasmoidThunder/This Silly Survival Thing We All Seem To Be Doing/@comment-26172389-20141002150942/@comment-3495717-20141123122159

It's F12. I tell people to do the PrtScr thing because nobody seems to rename the screenshots produced by MUGEN, meaning we have files upon files of mugen001.png, mugen002.png, etc. that get overwritten all the time.