The first thing to consider when selecting an image is Rights.
Rights - the ability to legally display an image without facing repercussions due to ownership over the image. Unless explicitly stated that an image is in public domain or available under a Creative Commons (CC) license, assume that the image has rights and at the very minimum requires attribution.
You also need to consider the appropriateness of the image.
Do not settle for an image that does not work for the intended purpose.
Use the images available from the links provided with the resources provided here to help you find the right images for your purposes if you feel that you must include an image.
Furthermore, do not choose images that are out of focus, crooked, poorly lit, or appear to be poorly aged--though a good vintage photograph or illustration can have a great impact.
If you use an image, remember that it will likely be the emphasis of the work. Use composition and other elements and principles of design to create a hierarchy, balance, and harmony to your design.
Do you want to use your own image or just crop something else a bit?
Editing an image is an easy way to make design different, and it will help you use images more effectively.
Most graphic design programs are not good for editing photos, so it is strongly advised to use a separate photo editing program before adding it to a design.
Great Tricks for better editing:
No need to do anything too fancy with a photo--adding text to an image in this process can prohibit readability and accessibility, save that for the design process.
Pixlr Editing Basics from the Elmhurst, IL public Library.