There is a clear guideline in copyrights:you can buy a piece of artwork in what media ever,but you're not allowed to change it or use it in a way the author has not licenced. Declaring a scene of somebody else as your own is a misuse. If you buy a painting and then you decide "oh,a green sky would be nice here,let's get some paint!",you are acting against international laws.The normal protection time for such cases is 72 years !!There is an old ,but still very true slogan for fine arts :"You can buy art,but you cannot own it ".Of course,not everything is art,but the licence agreements are the same, for the "Mona Lisa" or for a freebie from the download section.If the author tells you in the readme file ,that you don't have to give him credits, it is up to you to decide,but if it's written to give credits for using the item,you have to do so.Otherwise you act in an illegal way against license-laws !

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...yippi ah yeah or something like that...