You can use iPhoto, but if you don’t want to put your photos into that to work with them, try Preview. It’s in your Applications folder, and with it you can adjust the exposure and color saturation, resize images, make round and square selections and copy them and make new images. It’s easy to use to make simple edits with images you grab off the Web or your own photos.
You can also try Seashore, which is more towards a real image editor like Photoshop and less like a simple photo editor. It features gradients, textures and anti-aliasing for both text and brush strokes. It supports multiple layers, too. It’s a free download.
And you can also look at Apple to see the whole range of free and shareware imaging programs: Apple – Downloads – Imaging & 3D.