For a lightweight image editor, it depends on what you mean by "lightweight". If you're focused on size, how quickly it starts up, this may not be what you're after, but I recently bought Adobe Photoshop Elements 2.0 from Amazon.com, for $29.

Yes, $29. There are two special offers on it right now. First, it's on their list of Summer specials, you get a mail-in rebate of $40 (this ends on Sept. 3!). When I couple that with the $30 upgrade rebate for owning v1.0, the overall cost is $29.

Photoshop Elements 2.0 includes both Mac OS and Windows versions on the same CD, and is OS 9 and OS X compatible.