No probs let me answer:
For (1) add a special placeholder for “broken image”
For (2) add a different general placeholder for large image
For (3) and unknown, I am fine with “undefined” behavior. In fact I would crash the image post processor for local errors and make sure there is an error in logs.
HTTPS images should also be replaced with placeholder if too big.
Keep in mind, for now, this is optional default off behavior. Only add “too big” placeholders IF
replace large images with placeholders is on.
No idea about the placeholder images, just make sure whatever you pick is easily licensable. There may be a font awesome icon you can use, I would try that first.