up-sizing things can always be kind of iffy. if it's not a high-rez image then it's going to distort as you change the size.
if it's high-rez then you should be able to use any photo-editing program to resize the image. if not then you may want to just start over and rebuild it at the size you'd like it to be.
also i noticed that your sig is hosted on the edgegamers site an i know that if you add a sig that way it gets downsoized by the server so next time try hosting on imagebucket or flicker and useing the