Images are important final touches on assignments for the students or letters for parents. When you start looking for images to use, you need to be careful to avoid ones that are copyrighted. Many image sites offer to sell you royalty-free images (you pay once, and then the image is yours to use forever), but they can be expensive.
Luckily, there are a variety of sites that you can use to find images that are in the public domain - ones that are free for anyone to use, as long as they give appropriate credit to the image's creator.
Here are a few resources:
Sometimes you'll have photos or images that you'll need to modify before you can use them. These sites all give you the ability to do simple editing on images.