To be honest, iPhoto is good for simple editing such as cropping, rotating, red eye reduction, brightness/contrast, touching up, etc. This comes free with all Macs.
For a better paint program, you get Graphics Converter free with all PowerBooks and Power Macs. I don't tend to use that because I have Photoshop Elements 2.0, which is very good, but possibly overkill. (I got it for £50 ($80) and it includes both Mac and PC versions, and 3 home licences). However, this program is a cut-down version of Photoshop, but is also easy to use and has a lot of quick filters for adjusting photos.
iMacs, eMac and iBooks come with a paint program as part of AppleWorks but I'm not sure how good that is for photo stuff - probably only comparable to MSPaint on Windows.