As a web dude I have always found Stock Images the hardest thing to source for an affordable price. I am a member of ShutterStock which changed things for me. Instead of paying alot for a little you can join as a subscription and pay a monthly fee. This entitles you to 75 pictures a day at the moment which in my experience is more than enough for even a busy design department. You can download the stock images in high resolution also so banners and billboards are manageable.
So designers, whether web or print - have a look. ShutterStock