The El Torito Bootable CD Specification needs to be followed.
Nero will burn this right ONLY if it is used in the original ISO, (so the ISO needs to "be built" correctly), and IF the ISO is NOT burned as a file, but instead burned as an IMAGE... http://en.wikipedia.org/wiki/El_Torito_(CD-ROM_standard) Rick G From: "Brian Weeden"clipped
But, in your case you were using an image. The people that made the image should have already put the boot sectors into the image. So either they forgot (possible) or instead of creating a CD FROM an image you burned an image TO a CD.
