Rebooting does not solve the problem. Artwork doesn't change every hour. I can get it to choose new Artwork sometimes by killing and restarting the App.
"This appears to be a OSX/Yosemite bug"... I have to wonder. If you didn't test your OSX app on the latest version of OSX... Did you even test it at all?
I'm sorry you have been having problems. It was developed on Yosemite, and I did test. Unfortunately the bug was difficult for me to reproduce until just recently. This is a bug only some have been seeing related to how the OS caches images. I have submitted a fix, and the update is currently waiting for approval by Apple. Hopefully it will be available on the App Store soon.
"This appears to be a OSX/Yosemite bug"... I have to wonder. If you didn't test your OSX app on the latest version of OSX... Did you even test it at all?