The rendering code is too link to Java2D. I think it is better to separate the rendering part . I've also have experience in JOGL and i can create the JOGL (openGL ) renderer
indeed, i had the idea to turn classes like GameGraphics, GameImage, etc. into an interface and then add a Java2D implementation and OGL implementation
it appears you have some experience with OO design. personally i'm still learning a lot on that area

anyway, this forum is more for writing about yourself, e.g. how you got here, what you can do, etc.