[huuthi95]

KRAKEN.IO
With kraken you will do not need to install any plugins that the image displayed on your website will be automatically compressed significantly, for example if you have a photo with a space of 478kb will be compressed has 261kb image quality almost unchanged.
However it is not free but only allows you to try it
A detailed price list can be found here
Wish you have a nice day

[Kururin]

You could just use picresize, its an online tool that reduces file size and also crops the picture if you want

[Littlemaster]

I used manual image resizing using PHP GD, it is good but requires coding skill, allowing cache and resizing with HTML also works well, but if you are reading or browsing the page for the first time, it will decrease loading speed.

[Honey]

But why would you reduce the size of an image already in KBs?

Usually if you really have bandwidth issues the easiest and free of cost solution is to use something like jpeg.