Free alternative to Photoshop. You can get as complicated as you like, but it’s really easy to size, rotate and crop images, cut out parts of an image and drop it somewhere else, add text to an image, make banner ads, or put several images together. There is a simple tutorial, and anything more complicated you can learn from other enthusiasts online.
The only strange thing is that when you’ve finished working on an image and want to save it, you might have to click File/Export rather than File/Save.
Download Gimp for free here.