Large images in posts can cause horizontal scrollbars

Example: https://www.phpbb.com/community/viewtopic.php?f=456&t=2279391

We set the max-width to 100% for images in posts. But they have a border, which makes the outer-width 100% + 2px (which can cause scrollbars to appear).

We either need to set it to 99% max-width, or use a different CSS box-sizing rule.