To change the maximum size allowed for an image in a post on the forum, visit the admin panel and enter the Settings section. Select Forum Settings from the following menu. While in the Forum tab, find the field named Display Large Images. Select one of the following options from the dropdown.
Reduced Size to Fit Post Width
When an image in their post or signature exceeds the posting area's width, their image is scaled down to fit within the posting area.
At Full Size with a Scroll Bar
When an image in their post or signature exceeds the posting area's width, the image is displayed at the same size with a horizontal scrollbar.
This setting does not affect image attachments, which are already replaced with thumbnails when placed in posts.
After you've made your selection, your settings will save.