given a lot of people upload images bigger than 800by600 it might be worth getting a mod to resize the images so that it stops the images losing heads etc.
a quick google give me this
http://www.phpbb.com/community/viewtopic.php?t=347539
http://www.unframe.com/en/info/air-box-for-phpbb.html
its javascript based, so that needs to be enabled by users to ensure that the images show up. (such as me with no-script.) however most will have enabled it for the specific forum as otherwise it mucks up the ability to post messages (for example wouldn't let me add gaps between lines, took me a while to realise it was because I hadn't allowed that forum)

