It's just an image CDN. It's used by people running websites to offload the generation of thumbnails (and other smaller sizes) of large images onto a different server (the image CDN's servers). Images can be resized on-the-fly based on the user's device and screen size instead of premaking multiple versions of an image in some program like Photoshop.
It has nothing to do with how an image is presented other than to change the native size. It will have literally the same effect as just uploading a larger or smaller version in the first place. Nothing more.