Ok, currently there is no way to disable the img tag on the markup panel without editing the code. This is something we need to address for version 8 since it needs to be based on if you have permission.
In case the OP wants to know how to do that:
Comment out the section in templates/default/standard_text_editor.tpl that displays the image in the Markup Panel. Standard warnings apply; make a backup of the original file before editing anything.
<!-- Remove the image button from the Markup Panel
For some reason that if statement doesn't remove the image. Bug?
As for the first part of your question, currently img tags in posts is a global on/off thing for everyone. So the only way to allow it is to enable the img tags in posts option.
So, if you're going to fiddle with that in version 8, how about making an on/off option in Forum Permissions so the img tag could be allowed by Forum?