IMO, If you made the skin, it's yours. Since they are making money from their software, and if they are using your skin you should be entitled to something. Even if it just an acknowledgement. If you are happy for them to use your skin for free then that is your decision. If you were making money off making a skin with their skin components then that might change things.
NOTE: This is my opinion. I am not sure what the legal ramifications would be on this.