mod_rewrite for images uploaded

hello everybody

I want to know if it is possible, and if so, how is it done, that when I write the url of an uploaded image in one of my posts
for example:
/wp-content/uploads/2010/05/pic.jpg

calls a
/phpFile.php?path=pic.jpg

I want this only for jpg, gif and png uploaded files

I don’t know if it is possible, as the url of the file actually exists, and will load that image, but I want to do something over that image before showing it (watermark)

I tried with

RewriteEngine On
RewriteRule ^wp-content/uploads/([^-]*)$ /wphpFile.php?path=$1 [L]

but without any success

can you help me out?

thanks in advance!