When does "copying" a math diagram become plagiarism?
Paraphrasing Simon Peyton Jones here, credit is like love. Giving it to more people does not diminish it or you.
Even if you recreated an image yourself, there’s literally no reason to not attribute your inspiration for it (apart from your own ego).
The threshold for plagiarism is vague at times, but if you saw someone’s work, liked it and built upon it, give them credit.