I never tried this, so I do not know if it will work but you could try from the extension to do this:
NSBundle *mainAppBundle = [NSBundle bundleWithIdentifier:@"BUNDLE_ID_OF_MAINAPP_HERE"]
NSImage *image = [mainAppBundle imageForResource:@"IMAGE_NAME_HERE"];
You can also try:
NSURL *URL = [[NSWorkspace sharedWorkspace]URLForApplicationWithBundleIdentifier:@"BUNDLE_ID_OF_MAINAPP_HERE"];
NSBundle *mainAppBundle = [NSBundle bundleWithURL:URL];
NSImage *image = [mainAppBundle imageForResource:@"IMAGE_NAME_HERE"];
Again, I don't know if this will work; the extension may not have permissions to access resources in your main app. You can try though.