The easiest way to create a screenshot of your theme is to open up Alfred's preferences (invoke Alfred then press cmd+,). Click on the "appearance" button. Select your theme so that it appears in the Theme Preview window. Take a screenshot by pressing cmd+shift+4. Your cursor should turn into a crosshair at this point, and now all you have to do is to click and drag the space you want to shoot, and a screenshot will appear on your Desktop in a png format. Then, use that file for the theme.