Hi
Please check the error coming in log file "error.log" and browser error.
If there is no error, then it could be the case that you have set CSS for image as "visibility:hidden". When you do display:block to show it, it's there physically (I mean you can access it's properties but it's not visible. In order to have it show on screen, you need to set it's visibility to visible.
~kautuk
Kautuk Sahni