Our affiliate program at Diet-Patch.com has a full blown ecommerce and affiliate solution. Our webmaster ensures we have a variety of banners available. We are also open to any suggestions for new banners, a specific banner size and more.
That said, like your own question, we also find that some of our partners do not have the same offering of banners and promotional materials. When this happens, we'll create our own.
As far as stock photography, the most recognized would be Link hidden: Login to view , Link hidden: Login to view , and indeed google images as mentioned (but be careful of copyright issues here).
Make sure when creating your banners there is a call to action (CTA), and if you're creating an ecommerce website or marketing landing page that you have google analytics installed or similar to track your clicks and conversions to see if your banners are working.
Best of luck.
Director of Online Marketing
'the easy way to lose weight"