Modify Insert Image bb code tag

[Byggahus]

How do I modify the Insert Image function so that inserted images does not stretch outside the forum boundaries or that they are no more than 300 pixels width? I have tried to create a new bb code tag for this, but I did not sort it out.

[richpal]

I'm interested in this one because one of my users is using the IMG tag to post images into his thread (which is appreciated), only prob from my perspective is that the image is bigger than the text area and is stretching the forum width.

Is there any way to limit the max width of the image.

This is not about attachment sizes shown in attachments, rather images posted directly into the thread.