The option you want to look at is
Control Panel » Feature Settings
Max width for images used in posts?
Specified in px. Uses browser scaling. Leave blank to disable.
Yours was set to 400px I increased it to 600px just now.
It can be more just bear in mind if you make it to large it will cause scroll bars to be used in the forum when viewing the image.
Meaning you may have a bigger image but the user will need to scroll back and forth to see other content.
For an example see this test post at this board:
https://www.ubbcentral.com/forums/ubbthreads.php/topics/256629/test#Post256629
