Well, the pine (not tin) man page gave me a needed hint. Make a file in /etc called mailcap. In it, list for each mime type a method of handling said mime type. Calling external programs is possible. So by copying over a sample mailcap file from the tin docs directory I've gotten it so that jpegs can be shown by tin (by calling xv). Nice.
Of course if the image was just popped in as uuencoding with no mime "wrapper", then tin won't recognize it. I guess I'll have to find another newsreader. :/