Scratch

You make an interesting point, about using any material that was not created by you.  We have this line in the Community Guidelines:

Give credit. Feel free to make modified versions of other people's projects - just make sure to give them credit. One place to give credit is in your Project Notes.

But that really only addresses reusing material found on Scratch (the implied assumption is that the project owner created the material - not necessarily true).  We don't really address the issue of using material created by others on other sites in your projects.  That's because it's a really complex issue - and we are trying to keep our guidelines short and sweet. 

Whenever you use somebody else's work, be it a picture, a song, or printed material, you should think about whether you have a right to do what you are doing.  Sometimes the material has a copyright that allows others to copy (like Scratch does). Other times you might ask the author for permission to use their work.  Beyond that, you should think carefully about whether you should use it at all. 

You may be able to use a copyrighted work that doesn't allow free copying if you can justify it as an educational use under the "Fair Use" doctrine.  But it's a tricky area and you might want to avoid it.  We aren't lawyers and can't advise you on your specific rights for any given piece of material.  Your safest avenue is to use only your own work or work that is explicitly open for public use.

Beyond the issue of whether you have a right to use other people's material, it is NEVER okay to claim that you created material that you did not.  That's plagiarism and it is a serious offense, in most organizations.

Kind of complicated, yes?  And that's why it's not in the Community Guidelines.