It's the user's responsibility to ensure they aren't infringing on copyright. If you are producing creatives for pay, you absolutely shouldn't be right-click saving images and using directly off the web whether you got it from DALL-E or a Google search.

Who cares about plagarism? Plagarism is a made up boogeyman created by high school English teachers.

If the work is sufficiently composed of your own thoughts, the fact you used someone else's structure for part of it is not a problem as long as it isn't your entire work or entirely derivative of one work.

If I use a Coca-Cola bottle cap as structure in a sculpture, that doesn't mean I am infringing on Coca-Cola's copyright. I still had to mold my original work to work with it.