Found a source for these, but I don't know if it's the "original" source. The artist is identified as MonoLuna, but the link to their website seems to be broken/out of date.
Eeeyaaaah!!Huh? What?4. Sada-MarioShimuraaa!!
Behind you!!
"Shimura, behind you!" is a Japanese meme originating from the children's sketch comedy show, Hachiji dayo! Zenin Shuugou (roughly "It's 8 O'Clock! The Gang's All Here"). In one of the segments, some unknown threat would present itself behind one of the performers, and it was the job of the children in the audience to warn him by shouting "Shimura! Behind you!" The phrase is now used in any context to warn an unsuspecting victim of a threat sneaking up behind them.