If you're concerned about it, the easy solution would be to just redirect requests from different directories to a standard no-img pic ... To do it the way you are you almost have to be rewriting or saving the image repeatedly and which doesn't really matter, because a standard image will likely have a standard name, so you should be able to do this fairly easily:
1.) Save the image to a standard, simple location.
2.) Use a bit of mod_rewrite:
RewriteRule .no-image\.jpg$ http://www.example.com/no-image.jpg [R=301,L]
Check Out the Apache Server [webmasterworld.com] forum for more info on how it works or advice on how to change, edit or adjust to fit your needs ... It's a better place for a mod_rewrite conversation than here. ;)