Well as the post by Dr Drib is taken from the forum posting guidelines:
https://www.mobileread.com/forums/faq...ng_guidelines.
I don't think the guidance can be any more specific than the underlined text?
Don't post images bigger than 600x600 pixels.
It would be nice if the mods could just wrap over-sized images in spoiler tags so you have to click on the spoiler button for the image to appear, and send a PM and ask the user to make the image smaller. But that's possibly too much additional work for mods who probably have plenty of other stuff to fill their time.