I tried to add some custom ad html code to my full-image.php(zp-core/full-image.php). After adding the code it shows the error message
"Warning: Cannot modify header information - headers already sent by........."

How to add custom html code for this page without getting this error. For example. If i want to add banner ad at top of the page, how to do it without getting this error?

I don't know what that full-image.php file is about. This is surely no standard theme page.

Well, the full-image.php file is not a WEB page it is an image generator.

If you want a coustom page for full images you will need to modify your theme. Create a new page script with whatever you want and include an image source with the target getProtectedImageURL(). Change your image page to point to this new page and pass it the album/image information so that you can instantiate an image object to pass to the function.