Open your best image editor, if you're desperate use Microsoft Paint if you want to do something simple. If you are more ambitious, you should try to learn how to use GIMP. For Mac image editors, also see Free Pixel-Based Editing and Painting Software for Macintosh. GIMP works on a Mac, but compared to on Windows it is a pain. Set the size to 250 x 65 pixels so it will fit the logo space. This is a wide narrow rectangle, so if you're going to put an image in there, try to pick one that looks good in that shape of a space.