Those settings were added in 2006: https://github.com/phpbb/phpbb/commit/f95e730adf1d77e65cb4c408b49a62e2888b5f64

max_post_img_height
max_post_img_width
max_sig_img_height
max_sig_img_width

My guess is that back in 2006 it wasn't as easy to set limits in CSS, so they were implemented in PHP. The issue is that the image has to be accessible by the webserver, which is slow and fails often. When that does, the image either does not appear or the message is blocked. People who set that value in the ACP are not aware of that fact.