Originally Posted by jackie_w
... or another option.
If the image you want is not actually on your PC but is displayed on a webpage. In your browser right-click on the image and select Copy Image. Go back to Calibre and Edit Metadata. Right-click on the existing cover image and select Paste Image.
I found you could even "Drag and Drop" (non-flash) Images from Firefox (should work in M$ IE, but have not bothered to test) to the "book" cover in the editor.