And so to Image Compression!
Some file types compress their data better than others, jpeg giving one of the smaller sizes but perhaps is the more aggressive in throwing away data.
But is possibly still a good match for web needs as you can set, when saving a jpeg, how aggressively the compression should be applied.
There is also one more “trick” you might want to try in reducing the overall data size of an image and that is third party image compression apps. Some are free others paid.
Note these algorithms are not trying to change the number pixels but rather reduce the overall data contained within an image file. Although it is true image quality can suffer if they are too zealous in the pursuit of their work!
Here’s a link to a free web based service where you can upload jpeg, png and pdf files for compression, which I’ve found to give reasonable results for my occasional needs:
http://compresspng.com