Pick pictures with purpose
Your images should help achieve your site's goals — not just brighten up the page. If they don't convey something about your site, your unit, your service, etc., they'll simply feel like filler.
Check the rights
Whatever image you choose, make sure you're allowed to use it. If you don't know where it came from, the answer is no.
Keep quality in mind
Blurry, stretched, squished, or low-DPI images will make your site look amateurish. For tips on sizing images for web, refer to WMS article: 5 tips for creating picture-perfect web images.