The plagiarism rule doesn't apply to pictures because the picture itself contains evidence of its origin. If I were to post the below image, I am not trying to misrepresent it as my own creation. If you quoted my post, you would see this link that leads to where I got it from.
The link would only appear if you insert the link directly from the website into the image tag, but if I were to download the same image and upload it on some other platform like imgur, it will not show any link to the original source Here's the link you would get - 
Code:
https://i.imgur.com/Fssg7Uo.jpg
And here's the image:
Plagiarism does apply to images cause it can be stolen, but it's difficult to prove, probably the reason why it is not strictly enforced.