Well, you have to find and choose the method that fits your needs best. You never find anything that works the same in all browser. That is the nature of webdesign.
You can of course change the CSS to overlay an image on another using transparent images like gif or png (note if you use png24 older IEs need again special treatment).
Anyway, this all is a generel web technique problem and not a Zenphoto one. So I suggest to learn a little more about HTML and CSS.