There are a few solutions for something like this you can set your image width to 100% this way it will fill the avalible width then just allow the user to zoom.
you can also make a div thats 100% width place the image inside this and set -webkit-overflow-scrolling:touch; but this comes out a little funky because you can only scroll on one axis at a time.
Iv also written a plugin for fullscreen image viewing in iOS fullscreen mode that might help depending on your situation
www.uglymongrel.com and go under plugins its called jqm-image.js
let me know if you need any more help with this