Welcome aboard! And yes, learning Keyboard Maestro is a lot like learning coding, because there's so much to learn at the start. But you can do it!
Other people will probably have better tips, but here's some thoughts:
Your "fuzziness" slider is probably set too high. I'd guess you tried changing it to get it to work, but in this case, for the image you're looking for, I would imagine the default should work just fine.
It's possible that it's finding two images - the one on your desktop, and the one that is displayed in the action. Try collapsing the action (click the "v" in the upper-right of the action).
Finding images on top of desktop backgrounds can sometimes be difficult. In this case, since it looks like it's on a solid black part of the image, it may be OK (or maybe not). Something to think about.