OK that sounds good, so a larger image that are linked in from another site will not show bigger that examle 1920 pixels if that is set as above? is that correct understood or is it for attached images only?
Your attached and linked images should resize to be no larger than the width of your post.
Have a skim through this post -- which was used for testing exactly what you're asking
https://www.ubbdev.com/forums/ubbthreads.php/topics/320888/ubb-threads-wip-mobile-device-tests.htmle: if you dont have an account on UBBDEV.com, you'll only be shown the linked images, not the attached ones. But that should be just fine for the exact question you're asking.