What I do (and probably a bunch of folks do) is take a screen shot using the Windows/PrtScr buttons. That gets popped into Pictures/Screenshots directory. I then rename it to something I can find again later. I have an Imgur account and pop it there. When I post my score, I use the dropdown in the post that says "Other Media" and click "Insert Image from URL". From the Imgur site I click my posted screen shot and copy the Direct Link URL entry and pop it into my post by clicking Other Media / Insert Image from URL. I haven't checked to see if I could post the URL of Steam pictures. That might work, but I just use Imgur.