You cannot copyright ideas or concepts. The internet provides avenues for sharing of art and thought, but yes, includes the perils of theft. The concept of a house shape and text is not a unique concept. The finished work is the unique part, as it is unique to the name of the product and to the designer. Of course, even by adding a simply watermark and copyright notice in the image does not preclude anyone from "borrowing" any part of it. As for the lack of copyright information in the info file, like everything else, it can be modified.
In the US, the artist cannot sue for infringement unless he holds a registered copyright to that exact work. Registered copyright does not protect ideas or concepts. In order to protect a specific visual concept, scheme or identity, the artist has to obtain a registered trademark, which protects only that exact design.
Without registration, the only option to stop infringement is to send a written cease and desist notice in an approved format. Many web hosts and providers will take appropriate action to stop the infringement, up to and including threat to stop their client's services. This isn't always the case. Getting action taken is hit and miss most of the time, even with registration. eBay is one of the worst: if you do not have your work registered, don't waste your time. There are numerous "vendors" on eBay making thousands off my artwork as prints, vinyl wraps, cell phone cases, t-shirts, etc., and I have no power to stop them or collect any damages. Then again, a famous custom motorcycle shop used my art on a t-shirt in their online store, but removed it and stopped selling it altogether after I sent a formal C&D to their webhost.