Should projects credit off-site sources?

Lately, I have started to notice graphics based on uncredited reference images, though the issue is also related to uncredited code from other sites.

This came after I recognized graphics I had seen before on social media (Pinterest, Instagram, etc.), and vice versa. Many of these were detailed projects that the coder had clearly worked hard on - I would be impressed with them whether or not they were original images. But since the references were not credited, I felt tricked when I found out.

I think that not crediting your references is like not making a spin-off from the program that inspired you, or not crediting code from another project - as though it is similar to plagiarism.

As the guidelines have it:

It’s considered plagiarism if:

• A user pastes someone else's work into a new project, instead of creating a spin-off.
• A user pastes work from multiple projects without giving credit/links to those projects in comments at the top of the project.

Of course, I realize that some users may not give credit because they do not think off-KA links are encouraged. After all, links in webpage projects lead to a redirect page, and the guidelines above only say to credit code from other projects - not short scripts from another site.

My question: Should other sites be credited for code, and images credited for reference or inspiration? (And, for the sake of clarity, could I ask for the answer not only to be answered here but also placed in the guidelines? Thanks.)