LiveQuartz is a powerful free image editor. It is based on layers and CoreImage filters. You have the layers on the left and the filters on the right of the window. To edit an image, just drag it into the layers' list and it will be added as a new layer.
LiveQuartz has all the essential tools: The drawing tools (brush, lines, rectangles, ovals), the duplication tool, the eraser tool, the new background eraser, the gradients, the layer moving arrow, the text tool, the CoreImage tool, the rectangle and lasso selections, the hand, and the magnifying glass.
System Requirements Mac OS X 10.4 or later
PowerPC G4, G5 or Intel processor