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You uploaded it to LFGSS, under LFGSS terms of use, including:
"You give us unlimited permission to use your User Content and allow others to do the same for no fee. In legal terms this means that you grant us a worldwide, non-exclusive, royalty-free and perpetual licence to use, copy, reproduce, distribute, adapt, re-format, modify, publish, translate, licence, sub-licence, assign, transfer and sell the User Content anywhere and in any form."
Not really if you're the one selling it. I did the artwork for him as a friend. You can stop selling it and delete the artwork now. Thanks.