some photo management tools could be a help to you.
like f-spot or gthumb.
they can search you whole hard disk for images, but what they usually do is then import them into a single area. this means double up of images though.
a media server type app would be the way to go as it would just find and create a link list of where all the photos/images are.
but I can not think of one at the moment.
there is always the command line if you just want to find them.
sudo find / -name "*.gif" -o -name "*.jpg" -o -name "*.png"
this is just a quick and dirty regular expression.
check the man pages for a better one.