In my decidedly non-tech knowledgeable manner, maybe it has something to do with fixxed column widths. The column that the tag goes in may be fixxed at a certain number of pixels, and cuts off images bigger than that. Don't know exactly how to fix that, but it may be something to look for.
The icons have a fixed width of 15 pixels. Too wide icons such as Morrus new one will force the width of the icon column, and the forum will look horrible.
I would suggest the following:
Allow icons 20x20 pixels. This is large, but not too large. On the other hand, set the table cellspacing to 1 instead of the current value of 2-3.