After running GTmetrix speed-test tells me the following: "XYZ image is resized in HTML or CSS from 800x539 to 400x269. Serving a scaled image could save 112.1 KiB (75% reduction)." 

My problem is that all my images are hosted in an external server. And im talking about hundreds of images. So resizing/reuploading is not a viable solution, in other words i cannot modify the images.

What would be the most easy/viable solution to implement served scaled images?
Regards!.