Live Quartz is a nice free image editing app/download for Leopard. I consider it to be a cross between Photoshop and Acorn, w/ a very user friendly appeal to it.
At first glance it looks like your typical photo/drawing app, w/ familiar buttons such as: crop, draw, shade, etc. but by digging deeper you'll notice there is so much more. There are a ton features similar to Photoshop that allows you to add layers, effects, filters and so on. View Pic for filter/effects list.
Below is a pic I made using the Edges Effect and Circle Splash Distortion Filter.
For more info click here.