Clearly you were able to do so a year ago...
https://steamcommunity.com/sharedfiles/filedetails/?id=2614541872
Here's a how to reminder ;)
When you hit F12, Steam takes a screenshot.
Go to your Steam Profile/Screenshots and hit the Upload screenshots button. select shots taken from Valheim, from your games list, and upload the ones you wish to use.
Once uploaded, refresh the Screenshots page by hitting the Screenshots button again, your newly uploaded shots will appear there.
Now open one of them by clicking on it and you'll see at the top of the window an URL.
Copy that URL by highlighting it and rt click/copy (or CTRL+c) and then you can paste the URL (CTRL+v) anywhere you wish to post the shot.
Easy eh? ;)