Maybe you should try shareware registering services.
I currently use www.swreg.org and they are quite versatile and have a lot of options, but unless you use their advanced method (there is no fee involved) your forms will look a bit unprofessional.
These services deal not only with credit card transaction, but with software delivery by sending a download link by email when the credit card is aproved in real time. There are many protection methods involved, like IP checking or password generators. Check with each provider.
Check these urls for services comparison:
Also, you can try a service like ibill.com or others that offer recurring billing, where a customer pays a monthly fee and it is renewed each month o given period and the user can download all of your images (for this to make sense you'll better have a large collection). Of course, you'll have to set a private page where your subscribers can access your images.
For a way of serving a large collection, take a look at www.rebelartist.com They sell images starting from less than three dollars, and if you want, you can set an account and sell yours and they'll handle all the payment transaction. (hey! maybe that's what you were looking for :)
Hope this helps,