Actually, your browser does not download images on every request but automatically checks if it is modified using the If-Modified-Since header on request.
Web server returns a 304 Not Modified status telling the client "resource you are requesting is not modified, use your cache :)".

Actually, your browser does not download images on every request but perform a conditional request to the server using the If-Modified-Since header.
In case the image is not changed, the web server returns a 304 Not Modified status telling the client "resource you are requesting is not modified, use your cache :)".