What issue are you having with them? Do you mean the signature part that goes under your posts or the section where you have "WTB +10 vs plants..."? If the former, then it should be just as easy as adding any other image. If it's the latter area, then you might require HTML image code instead of BBcode. Not sure.
Just make sure any .gif image you want to add (I'm assuming it's going to be an animated .gif) is light on size so as not to unnecessarily weigh down thread loading.