Jonesy - 9:30 pm on Mar 11, 2012 (gmt 0)

linux:
I generally use the web site layout ala:
...../documents/
...../documents/images/

On my workstation (using the command line) I enter the .../images/ sub-directory
and:

...../images$ for IMG in * ; do echo $IMG ; grep -l $IMG ../* ; done

(That's a lower-case "ell" for the grep switch.)
Any image listed without any following lines of files noted by grep is a file not to be found in any document: html, php, css, etc., usw.

If you wish, you could get more elaborate with the -r|-R (recursive) switch for grep.

Of course it works only for static web pages....

HTH,
Jonesy