Hi Laura!
Stock photography is really useful - I use istockphoto all the time - but I always get pay-as-you-go credits.
Advice:
- Never use an image as supplied because you'll never know when this same image appears elsewhere!
I tend to get their images and work with them to create a complete different version.
Once I bought an illustration for a school brochure, a week later, the same artwork was used for the strip-club flyers down the road
~Peace~