The build-in graphical screenshot tool in OSX is crappy because it only save the file in tiff format. You can use this commands to maken a screenshot and save it to PNG format.
Key CombinationResultCommand+Shift+3Capture entire screen and save as a fileCommand+Control+Shift+3Capture entire screen and copy to the clipboardCommand+Shift+4Capture dragged area and save as a fileCommand+Control+Shift+4Capture dragged area and copy to the clipboardCommand+Shift+4 then Space barCapture a window, menu, desktop icon, or the menu bar and save as a fileCommand+Control+Shift+4 then Space barCapture a window, menu, desktop icon, or the menu bar and copy to the clipboard
You can also download and install the freeware tool SnapnGrap from http://www.yellowmug.com 