You go to options/configuration in csTimer, then you go to display and in display you select background image. If you select "manual" then you'll be able to put an image's url link to set a custom background of your liking. Or you can also select "upload" for a image that you have saved in your pc. All this is circled in red.
It has way more features too.
For example, if you want to practice an OLL alg, you can go to their training section, and it will give you a scramble to the case, and you can time yourself. You can also race other cubers online.