Does image block save internet bandwidth usage? I have a data capped plan from my ISP ( 5GB at 2mbps and thereafter 256 kbps / pm). I doubt if the addon or other similar addon actually saves bandwidth.
When a request is sent to the server, the server sends out whatever page it's requested to serve with all its text and images etc. So essentially my ISP has made his pipe available for the data to reach me thus he would count those bytes under my data plan. When the data arrives it's all first stored to my browser cache (folder) area which means all the data has actually been received by me/computer using my ISP's pipe.
The browser then fetches those data from the cache and displays it. By hitting the stop button or blocking images via addon I am just choosing not to display the data which would remain in the cache or eventually be discarded if still on the network pipe after a timeout limit.
The point is the data request have been completed by the ISP and so the data would be metered and thus using addon such as image block or hitting stop button while page is loading does not in any way save internet bandwidth.