ImageWell is a free image editor for the Mac.
From their website:
ImageWell is the popular image editing software for quick manipulation and upload to an iDisk (.mac account), FTP server or other web server. Whether you are adding images to your documents, editing offline images, or posting photos to the web for others to see, ImageWell will do the work for you. Drag images in and out of the well, resize, crop, shape, rotate and add a watermark, border, or drop shadow. No need to launch multiple applications to add text, labels, arrows, circles and squares to your image. Add a thought cloud or talking balloon. A few simple clicks, copy, paste, and send them off to your web server instantly. At the click of a button the image is sent and a handy URL is copied to the clipboard. Just like that, it doesn’t get any easier!
I used ImageWell quite a bit this summer to edit screenshots for the Tech-Ease website. I used ImageWell to place arrows, callouts and text boxes on my screenshots (which I took with either the keyboard shortcuts built into OS X or the Grab application found in Applications > Utilities.) ImageWell allows you to take screenshots as well, but I needed the delay feature of Grab to take screenshots of open menus.
The program really is very easy to use. I like how you just have to drag an image into the program’s window to get started with it. Scaling an image is also very easy, all you have to do is use a slider until the image is the size you want. If you prefer, you can change the save location so that the edited image is saved to any folder on your computer when you click on Send.
ImageWell is a great complement for iPhoto.