Stock images can be cheesy, staged, unnatural, cliche, especially when it comes to getting a shot of "real" people with real emotions. Try Flickr or other image sharing services as an alternative to stock image sites.
Here is a great, spontaneous and real image that caught my eye today. Look at the emotion in the girl's eyes, great light coming from below.
Original (in larger size) here on Flickr, picture taken by Studio Cougar. Always check copy right and license restrictions before using Flickr images in your presentations.