And on Mac you go to Finder>Library>Application Support>Minecraft>Screenshots>and your find you picture
This thread should be a sticky. Too many complaints have been dismissed due to lack of evidence, cropped screenshots, etc.
Updated this in various small ways since it has been bumped... I have a Mac now, so I've added more clarity on how to use a Mac to take/find screenshots. I currently don't have programs or info on things like Mac's screen grabbing tool, Windows Snipping Tool, gyazo, prntscrn, puush, etc. Personally I don't support the third-party programs (they are not secure and it encourages evidence that cannot be used), but if people would like information on OS's default screen grabbing tools, I can add that to the OP.