matkatmusic provided this code in the Discord applications server, it works! Of course, this is flipping the entire graphics context, not the individual image, but this works for my purposes.
// horizontal flip, to be used in a paint method
g.addTransform(AffineTransform(-1, 0, getWidth(),
0, 1, 0));
If someone gets around to it, please provide code on how to flip an individual image easily, I’m sure it will be valuable for future readers. Or maybe there’s no simple way… I can imagine either creating a “FlippableImage” custom component class or drawing/storing a flipped version of the image during runtime to an image member.