When you press F13 to take a screenshot in StarCraft II, the game saves it as a JPG file to:
/Users/<your OS username>/Library/Application Support/Blizzard/StarCraft II/Accounts/<your SC2 account ID>/<some subfolder>/Screenshots(I’m not sure what is this <some subfolder>, the name is inexplicable).
To reach the destination quickly, in Finder press Cmd+Shift+G and paste the following:
~/Library/Application Support/Blizzard/StarCraft II/Accounts/Press “Go” and check the subfolders.
I thought I’d post this because the path in Blizzard’s article is wrong (as of writing).