Shift+Command+Ctrl+4 on Mac copies a selection of your screen that you select to the clipboard. Shift+Command+Ctrl+3 does the same thing but with your entire screen. This format is an image and can be copy and pasted into various Mac applications such as Preview, Mail, etc for very quick screenshots. However when you try to do this with Pinta it says that the file in the clipboard is not an image, when it definitely is.

For me, on Mac OS X 10.10.2, 13-in Mid-2011 Macbook Air, this comes up with an error.