Here are some free programs to clean up still images before dropping them into your edit timeline. The first is Gimp, "a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring."
Gimp is another commendable example of the open source community working to make useful applications freely available. They also prevent predatory development and distribution practices by software companies.
Another free program is Photoscape, "the fun and easy photo editing software that enables you to fix and enhance photos." Google also offers its own free application, Picasa. Finally here's a link to an article which lists a number of other free programs for image editing.
Most of these applications allow you to edit, resize or crop an image in order to match the screen size and resolution of your video. Each also offers an excellent opportunity to get your feet wet in digital image editing without dropping any cash.