PHP Image Functions

I recently tried to make images do the same like this site

http://danasoft.com

But, I donnow why he could use .jpg as the file extension.
I'm only stuck with .php
Is there like a way to mask the file type?

Actually, you could make your php-file send out the http headers content-type and content-disposition. I think in PHP you can do it like this:

header('Content-Disposition: inline; filename=image.jpg');
header('Content-Type: image/jpeg');