The easiest way to offer an item for sale is with PayPal. 

To add a Buy button, go to the image gallery > Image fields, and turn on the For sale and Price options.

You can then edit the price field for each image.

This will automatically display a Buy button next to any image with a price that is checked as For sale.

Money is sent to your PayPal account via the email address entered in the Site settings > Site content > Your email.  If this is not the email address associated with your PayPal account, you can add it as an additional email address in your PayPal account.