You are working on countless images on your Mac and if you want to find a specific image, you often select an image on your desktop and hit the Space key. The image is shown in a window for a preview. This is the QuickLook of the Mac OS X. There is a plugin for QuickLook to show you some more information.
Get qlImageSize and install it on your Mac. If you hit the Space bar this time, the window with the image pops up and the image dimension and the file size will be shown. Fantastic. I can not work without this little feature any more.
Get this plugin here: qlImageSize
Info: qlImageSize on GitHub