How to hide a portion of an image in html

Before you tell me the same thing the whole internet has told me, which is to use overflow: hidden;. But this poses a serious issue on mobile, http://latest.tritinia.com/from-the-grave-and-back/ch-47/ is something i used to test it, it works like a charm on desktop but on mobile, it just doesn't work, i tried making a separate mobile site to fix the height for the mobile then noticed each mobile device has a different browser resolution. PLEASE HELP!