NSBundle that is available for macOS screensavers isn't the same as the usual
[NSBundle mainBundle]. Instead one needs to use something like this when dealing with the
NSBundle *bundle = [NSBundle bundleForClass:[self class]]; // and then load an image, or something NSImage *image = [bundle imageForResource:@"gopher"];
Maybe this is documented somewhere, but I seem to have missed it until I came across this helpful comment after much frustration.