You can add images from your Media Vault to a Gallery and set a purchase price and/or customization price for each item.
If you every item will be the same price you can simply set this on the original Gallery and not per image.
Purchase Price = The price a user will pay to simply add the image to their account.
Customization Price = The price a user will pay to have the image customized by you (i.e. add text and/or effects to the image).
Please note images displayed on the "Select from Media Vault" page are automatically resized and are not the size they will be displayed when a user views your gallery (they will be full size).