Where "myimage.gif is the name of your image file. Make sure WIDTH and HEIGHT match your image dimensions. The image may not be visible in preview mode. You will see the image in the menu after you export the .js file and copy your image files to the same folder, together with your .js file.
To see the images in preview mode adjust "base url" to point to your images folder. To do so choose "Select Base URL..." command located under "Menu" menu, and put full path to your images (with ending backslash):