Paypal shopping cart buttons work well enough.
Create a button for each size photo - 6x4 10x8, 16x12 etc.. on your website. The customer then just needs to input the ID of the photo they want printed and click the relavent size button. The paypal screen loads in a window showing your order. If you have business account setup in Paypal then Customers don't even need to register
Once an order is placed you receive an e-mail from Paypal with the details and you sort the printing yourself.
See this example :
click
Ideally you'd want the printing included but systems that handle everything cost a lot of money and it's s only a viable option if you're photo sales are very high.
For example :
Thirdlight
I used this for a year and it's an excellent service - It handles orders for photos and printing through Photobox. It can be integrated into your website so it looks very professional to customers.
However the setup cost is £300 and £70 a month thereafter.