As you can see in the first image, when inserting an image by "URL," the size limiters are not respected. The second and third screen shots were inserted "by file." Notice the third screen shot shows the image appearing correctly in the preview.
There's an open bug case for this from October. I added this thread to the comments.
I've been frustrated by this problem for some time.
As a workaround, you can manually edit the HTML and add width=550 (or whatever width you want) after the img tag. It appears that the "Max width" field inserts a style parameter (style="max-width: 550px;") which gets ignored by the forum software when rendering the image.
