Generally speaking it is NOT legal to modify something and claim it as your own (Copyright Infringement) without crediting the Author and still if anyone says anything about it or staff notices you better be able to show proof you have permission to use it or it will be pulled and possible Banishment for you, especially if it's a habit....unless it's of course it's in the Public Domain or stated for public use by the originator

Remember...No matter where you go there you are