Valve's artists display digital gallery as part of Steam Community update
Valve are currently beta testing a selection of additions and changes to Steam's Community pages. Not only have they separated out general community activity from your own contributions, and created a Community Home as an all-game version of the Game Hubs, but you can also upload artwork for any Valve game. It's not all charmingly crude MS Paint scribbles, though, as Valve's artists have created an installation for this digital gallery, featuring concept, prototypes and posters for everything from Portal to Dota 2.
Artwork can only be uploaded for Valve games right now, with the rest of Steam's extensive library set to follow in the future. To use the new features, you'll need to opt-in to the latest Steam Beta Update from Steam's settings menu. You can also view them in-browser by joining the New Steam Community Beta group.
Steam has also updated Workshop entries to provide more detailed stats to creators, as well as new tabs for each Workshop item, letting Subscribers create discussions and view a full update history. More details of that update here.