Honestly, screenshots like that full of photorealistic 3D scenery should be JPEG, unless you absolutely need to preserve every single pixel 100% accurately. (If you do need 100% pixel accuracy in a particular area, cropping out that one area is an option.)
Of course, that doesn't really answer your question of how to best (convert and) upload such ...
Instead of uploading your newly edited picture, save your copy in case you want to edit again in the future, then use something like Windows Snipping Tool to do a screen grab of what you are wanting to upload, and use that much smaller flat image.
I found it's easier for me to just share to some integrated social network and then insert picture using direct link from it. Previously it was possible with Facebook or Twitter, now I can only see Twitter option.
No need to use any usb stick;
Quality is automatically reduced to some jpeg.
Maintaining one extra account;
Quality is ...