go to Window- Preferences- Shelf Editor--

Once there, select the self that you have put the x-ray icon on, scroll down to the correct button( you should see in the line of shelf scripts the x-ray script that you inserted, also if your having trouble finding it, they're listed in the same order that you see them on the shelf.) Once selected, hit the change icon button and direct it to the path of the file you want to be your icon.

you can also make your own custom icons in photoshop, but make sure to save them as the right type of file or maya will not recognize it. Check the other icons to see what type of file, it's also a good idea to open one of the existing icons, edit it, and then save it as something else.

good luck