Hop, je serai moins bête l'an prochain :

PNG is already a compressed data format. Compressing it with GZIP is not likely to decrease the size, and can in fact make it larger.

The need for X-Accel-Redirect (and it’s sibling X-Sendfile) comes from two distinct requirements
The need to deliver large files.
The need for those files to not be available to the public.