If you need to do bulk image manipulation, using a Mac with Leopard makes it easy even if you don't know (or even have) special image editing software. In Leopard, you can use Preview to do bulk manipulation of images.

To do this, open a Finder window and select all of the files that you want to manipulate. Then right-click and select Open With -> Preview to open all of the images in Preview. Once Preview opens, select Edit -> Select All from the menu bar to select all of the images. You can then go to the Tools section of the menu bar and apply any of the effects available there: you can adjust size, color, rotate and flip. It's obviously not as powerful as something like Photoshop, but for simple bulk changes that you want to make without any fuss, this is a good way to do it.