For some reason, it has to do with the code you have in your signature. Let me show you.
[center][img width=296 height=200]http://www.majhost.com/gallery/Cospas/MLPShop/HappyPonies/starrynightsgalaxyhp.gif[/img][/center]
Do you see where it says width=296 height=200? That causes it to stretch to those dimensions. If you remove it and just have it with [img] only (no width/height), it should go to its default size.