After several years of use (even if happens with every Gecko/GRE/Mozilla browser), what happens is just a mystery.
I only have thoughts about it and some point that depends on how the page display the images. Let's explain it.
If you visit a page, that, for example, display pictures through CSS instead img HTML tag attributes, and this is used for background images, and this happens very often lately, it happens.
Instead of display, I should say how is the image declaration or how it is called from HTML.
After the parenthesis paragraph above, what happens is that makes the browser to call the correct placeholder or not.
And even what I said, my thoughts are not clear
Sometimes happens, some not. Why? Because looks like the quicker the page renders, the more probabilities we have to see the red dot.
And, also, that now javascript is used often to load images, and with the new faster javascript parser... errr, well, imagine.
Anyway, a f******* mystery and thoughts. You can search all the web and you'll find the same stupid answers. Including in the Mozilla forums.