How to protect yourself when working with opensource?

For me personally, this is a big issue. I grew up around the nuke community designing themes. I love to design phpnuke, zentri and other opensource themes. But within the past 6 months people have been legally redistributing commercial themes people like kenetix, xtrato, and others are making...They are nuke theme designers. The People who do this claim to be gpl hunters. They are saying anything made for opensource is under the gpl license and can legally be redistributed. My question is how can i protect myself from this legally. What can i do to prevent it from happening? I heard that i could possibly zip the code and images seperate. But i need a better way.