Rather than expecting you to know all of the different file extensions and file types, or be technical enough to understand exactly which files to back up, Microsoft has made it simple by providing checkboxes for categories of files.
For example, you don't need to know that a graphic image could potentially be a JPG, JPEG, GIF, BMP, PNG, or other file type. You can simply check the box labeled Pictures and the Backup and Restore Center will take care of the rest.