The easy way of getting your xbox screenshot onto you pc is this site here, https://xboxclips.com, type in your gamertag, search, at the top right, click on screenshots, click on the one you want, and on the right hand side you will see download png, click on png and done.
the other way is, go to your screenshot on your console, upload to onedrive, get on your pc, load up onedive, click on pictures, xbox screenshots, then you can just move it over or upload from there to whatever site you want.