Why do PNGs with embedded HTML links not open on iPhone and only allow to save as image?
I'm sure that the datatype of IMAGE:PNG isn't set up to have clicks that have hrefs in them isn't set up to be detected and processed in the viewer on the iPhone.

Certainly appears to be a limitation in the image viewing code.

If you could save that image on your iPhone and send to your desktop over email, you could check if the href was being removed somewhere in the process, but I don't see an easy way to make this work.