I don't know if this has been mentioned before or if this is widely known but it was new to me; I came upon it by accident. You can save any graphical clipboard contents to a file if you have Preview open. Hit command-V inside Preview and a window will open up labeled Clipboard 1, showing the clipboard contents. Hitting Command-V repeatedly will open windows labled Clipboard 1, 2, 3 etc, each with a copy of the clipboard. You can then choose Save As... from the File to save the newly created image file.