Using a local(!) plugin to reduce image size and quality requires you to first download the large version. Thus you can not reduce bandwidth usage this way.
The only way to use less bandwidth is if:
- Someone else downloads it all, then reduces the size and you download the smaller images. (Some networks do this when you tether a mobile phone to your desktop)
- If the site offering the images also offers smaller images.
- You turn off image loading in a graphical browser, or
- You use a text only browser such as lynx, links or w3m.
Note that you can be your own somebody else from bullet point number 1. All you need is:
- A place with no bandwidth problems / limits.
- A server to run software on.
- A proxy installation configure to pass all traffic but which reduces image size.
Note that you can also use that proxy do do other things with the images.
E.g. see this XKCD comic where the images are not reduced in site, but turned upside down.
More information on how to do that can be found here.