I have 7 customized smiles added to my vBulletin forum. 4 of these turns up as red crosses in vBGallery - which looks pretty bad from a layout perspective.
Likely the image path just needs to be updated to be fully qualified.
If you modify the smilies in your ACP the path is likely something like "images/smilies/abc.gif" so you just need to make it fully qualified like "http://www.yoursite.com/forums/images/smilies.gif" instead.