It is mandatory to credit/mention if the vendor says it is. The creator (vendor) can set what the copywrite rules are on thier products in thier terms of use txt that comes with each product when people buy it. If that is not noted in the terms of use, then it's not an issue. If it is, it is. That's just simple copywrite law. Courtesy is another issue all-together. I for one, credit every single item that I did not create myself in at least some way. It's about integrity and respect for your work and others.

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