If you are really serious about your question Christian, you have a loooong way to go at the dark side of Photoshop.
I think it is not common for an artwork with no watermark will end up on t-shirts. Otherwise, everybody at the International F-104 Society would have Lieuwe's amazing F-104 profiles printed on their underpants. Well, I do not have too much data to prove the opposite though.
People steal your artwork to make a few dolars profit. These are "6 USD men". You can find them selling reprints of already published Bf 109 photos at eBay, while they claim that they have the original negatives. One of them in US got a penalty in the German court for violating copyright laws but even this did not stop him. He is still the No1 Bf 109 parts seller at ebay. Let me tell you that no watermark can stop them.
Well, this is my 2 cents worth. Profiles are yours (and underpants are mine).
Lieuwe, if you are reading this, I have just completed the instrument panel of D-8058. Can we please proceed to complete the profile?