If your site is looking a bit tired and perhaps rather dull, using a few images to break it up is a great idea. Typically, stock images can cost a lot to licence for online use, however a number of websites offer good quality stock images for free.
One of the most well known and a site I have used several times is Stock Exchange. The selection is good and the quality is certainly good enough for most projects especially if you spend a bit of time editing the images for your specific application. Make sure you check the licencing terms of any image you use as sometimes the photographer will require you to tell them where its being used (usually for interest sake more than anything else).
Another great source of images is searching through Creative Commons licenced shots. Flickr’s advanced search provides options towards the bottom of the form, for searching only those images which can be licensed (via Creative Commons) for use on your website.