You can re-size an image
The normal BB code of
[img]http://www.picgifs.com/graphics/h/happy-birthday/graphics-happy-birthday-501480.gif[/img]
will produce:
If you add a height paramater like this
[img height=300]http://www.picgifs.com/graphics/h/happy-birthday/graphics-happy-birthday-501480.gif[/img]
Then you will get:
Although, gif's are best viewed at their original size.
If you add both height and width options you can make a real mess....