Still debating as to whether to give users access to high res images? What are the potential advantages/disadvantages of doing so?

What exactly do you mean by high-res? 300 ppi printable or just large wallpaper size (ex. 1600x1200)?

Advantages: They can buy the pic from you and print it on their own, without you getting involved.

Disadvantages: They can get from you and print it on their own, without you getting involved. Also, eats your bandwidth faster.