Since it appears the original work was not contracted with the intent that they would own the images (which would have required a much higher up front fee regardless of the end product), it sounds like a licensing deal is probably the best way to go.

A license allows you to provide the high quality finished product, but with restrictions on what it can be used for based on what they have paid. This allows the business to do what they need to do with the photos but also allows you to protect your brand and reputation and get value from their use of your photo.

Ideally, you should find out what their intended uses are and outline what is allowed for the image in a license agreement, including limits on the size of the audience. For limited marketing purposes like you described for a small martial arts school, probably $30 to $60 is good if you are doing this part time. Full time, I could see it being more like $50 to $100. (Note these estimates are only for the image licensing and not for generation and also are based around the idea of licensing multiple images.) If they agree to meet your quality standards for printing the image and give you photo credit, then I could see going much cheaper. Say, something around $15 an image for part time, but I'd make sure that you keep quality control over the printing if your name is going to be associated with it to ensure your brand is protected.