I doubt your Mac can't handle Photoshop. Why do you think this? If you're talking about your G4 Mini, then you're off base by a mile. 512 Mb RAM isn't generous, but for native OSX/G4 it is adequate for Photoshop, although 1 Gb would be better.
If it is an old Mac, look for an older version of Photoshop. I have PS 7 on my G4, and PS 4 still runs fine on my G3 under OS 9
Graphic Converter is a general purpose tool that can do some but not all of the work. An older copy of Photoshop LE, Photoshop Elements, or Imageready may work, too. Macromedia Fireworks is also an image editor.
But I don't think you're going to get Mac-native GUI, semi-pro almost PS quality image editor for free.