Via Cult of Mac
Update: Currently not working on Mavericks.
I can’t even count how many times a day I have to check image dimensions, image sizes, and my current solution currently requires using the Finder to see the document details about each file. That gets too time consuming, especially when I’m doing it all the time.
I found this great little QuickLook plugin that solves this problem for me. Using the built-in awesomeness of QuickLook on a Mac, I can quickly and easily see the size and pixel dimensions of any image.