Serving compressed images

How can you tell?

[directrix]

Is it possible to tell, by looking at the HTTP headers or otherwise, whether a site is serving gzipped images?

(I realise it doesn't make much sense to serve compressed images, in that they should be held on the server already compressed.)

[encyclo]

If you want a simple check, load the image directly in Firefox and select Page Info (Ctrl+I). If the file is gzipped you should see a response header "Content-Encoding: gzip".

Of course, the server should not be gzipping images, especially as JPGs are already compressed.