The recipient pays if they don't have a messaging plan. You can send an MMS to a US T-Mobile customer by emailing firstname.lastname@example.org (if their phone number were (111) 555-7777), so that's how I do that.
I don't know, all the carriers over here have a way to send a picture to a phone using an email address. It would be very nearly impossible to block his app: They'd probably have to block individual users for receiving too many emails. Not an attractive option I wouldn't think.