You should definitely contact the artist and Valve about that. I'm a workshop artist myself, and it's important for us that the art posted is original. If anything, maybe just ask for a 30% cut of the profits from the sales. It is worth your time to pursue this.
Is it acceptable that your knight design seems to have been used by someone for an armour model in the workshop for DotA 2 (in which the one who submitted it would get money out of it if it's accepted)? :/ steamcommunity.com/sharedfiles… Incredibly awesome piece btw, that is one fine helmet