You may want to think about adding a custom IMG code, that resizes the image to X dimensions. This is the html code to resize an image:
HTML Code:
<img src="http://www.cjmmedia.com/images/styles/freeminium/banner.png" width="1022" height="200">
You could make that into BBCode.
Or you could enable attachment thumnails, then let your users upload images to your server.
To enable attachments, go to the AdminCP > Settings > Options > Message Attachment Options > View Attached Images Inline > Yes, display thumnails > Sort out various sizing options > Save.