First time I tried this, but dragging an image from a webpage opened in Firefox does not seem to work.
That being said, I found this post where it’s mentioned that this functionality was implemented, so I’m reporting this as a bug: Drag and drop image (from web) / paste image link -> embed image - #3 by edwin
Additionally, I tried dragging the same images from the same webpages, but using Edge, and that worked.
I think this is where I noticed the issue: How to Craft the Perfect Daily Schedule (According to Science)
I just tried again, and it seems to work if I open the image, and then drag that, so it might be something on how the page is made. Nevertheless, maybe it’s something you can look into, as it seems to work just fine with Chromium browsers.
P.S.:The first image works just fine. Try with the rest.