Graphics from the server to the mobile application. Keep a mountain of sizes or spit on traffic?

Question

Mobile development

As an example, let's take a prototype of a mobile application in which the user gets the opportunity to upload a photo to his album.
At the moment, after uploading the file, a reduced copy of the required size is created on the server, x2 copy for phones with a retina + the original.
Please tell us how to properly organize the work of Image Resize, what sizes can be taken as a standard? What rules are used when uploading graphic files to a mobile application?

Answer 1

AJAX->We load the required size from the server

Answer 2

Keep one size, resize images with nginx analyzer address. For example
example.com/foobar_300x200.jpg