How to add the ability to add photos from social networks to the site?

Question

JavaScript

Good afternoon! I am writing a site in Python-Flask, I want to add the ability for users to upload photos to the site not only from a computer, but also from social networks.
1) I didn’t find anything worthwhile on the Internet, point me in the right direction).
2) Once I wrote a site on localhost (php -S localhost) in PHP, I made a "share" button there, since I easily found the JAVASCRIPT code for this. Only now the button worked, but nothing appeared on the page in the social network. Can a social network (VK, for example) block data from inaccessible sites, since the server, I repeat, was on localhost`e. Can there be the same problems with point 1? How to solve the problem without buying hosting?