Thank you Cyberek, this worked great! I found the demo images in the template directory -- for example gkcrop.png. I was able to make similar files that are 2 images stacked in the single .png file.
I uploaded the image files to the Media Manager. I put the css code you suggested in the Template Manager / gk-simplicity / Advanced Settings / in the Custom CSS Code text-box. I put in 3 CSS code snippets to replace gkrocket.png, gkmobile.png, and gkrocket.png.
At first my files were too big and looked funny, but I shrunk them to 126 × 252 and they worked fine!
Thanks for the help!