I would recommend you to use some open source e-commerce application for this.You can setup your pictures as products and people can buy it.oscommerce is a good choice to start if you know little bit PHP. It has got wide range of add-ons which u can use to integrate the payment option and all.
Reasons you should consider such products
1) Already tested by many people
2) You can save lot of time.just download the source code and modify it as per your requirements.
3) Wide range of addons and community support