This afternoon I came up with a handy little bookmarklet (or favlet) that uses the site dummyimage.com to create a place holder graphic. Simply drag the following link to the button bar of your browser:Dummy Image
Here’s the code:
When you click the bookmark it will prompt you to input image dimensions. Use the format, WIDTHxHEIGHT (for example 640×480) and a new window should open up with a downloadable GIF with those exact dimensions.