Mobile Maptip Too Large, Running Off Edge of Screen

On our handheld and tablet HTML5 viewers, our embedded photos inside a maptip are too large and are running off the edge of the screen. I tried the 'PanToPoint' function and that helped a little. Can I force the size of the maptip window either in CSS or the JSON file? If so, where and how do I do that? Or do I need to make the .jpgs we are hosting smaller?