Omg guys you're over reacting about steam viruses now? What year is it?!
Firstly lets talk about the lvl 0 random dude that wants to friend you that is probably a phising bot.... Oh wait it is! He will say something like Hey, for some reason I cant add you, can you add me? *insert fake steam community link here*
Last but not least, the .png bot. Its basically like the phising bot except instead of a fake steam community link, it gives you a picture type link. Do not click it as it will download a file which hacks your steam and says the same message to your friends without your notice.
Oh and the Give me your password for free games guy. That guy is stupid.
Any links you get from dodgy screen sharing hosts are most likely viruses. I have received reports from plenty of people talking about this issue.
The screenshots in case are actually .NET applications, disguised as .png or .jpg files. Windows is smarter than you think, and it actually opens the file as it is intended, using Windows. It's not an image, it's a .scr file which was used as a screensaver file back in the days.
The screensaver file contains a .NET application that uses the Steam API. It automatically trades all your stuff with some random user and they do this to steal (mostly) your DOTA 2 or CS:GO keys. As soon as you opened the file, you will send this to everyone in your friends list who is currently online.
You can not blame him though, a friend of mine has received a link like this from a really good friend of him so he'd never expect it was dodgy. He clicked it and he lost his entire CS:GO inventory (worth $300+).