Th problem is in the handling of the image size. Include a width attribute – which of course actually makes it into valid HTML – (well the width AND the tag closure slash_ – and it should be fine. Use the TinyMCE image insert dialog and this will do it for you.
Now – having said that – I am wondering if we have been a bit too enthusiastic with the checking and I have a note already on hand to look into this. But the missing width attribute is actually the start of the problem – followed by the failure of an attempt to discover that width before saving the post.
But basically yes – I am looking into it.
Did you try the subforum icon code yet? Waiting to hear back from you on that…