I got a RL photo of a street sign covered in "Derp" stickers (and a clown face) but it's 1.1M (.jpg) and gog caps at 500k.
What's the best way to shave it down to post it (not in the current game, obvs) without too much resolution loss?
Just open it in any photo editing program (GIMP, as cristi suggested, is free) and save it as JPEG again. When it asks for quality just lower it until you get a file size that will fit. If that loses too much quality or doesn't make it small enough then resize the image first (if it's from a modern camera or even a half decent phone it will probably be massive)