As far as making your own sig, I'd lay off til you're a little more computer proficient. And you'll most likely need some photoshop skills as well to make a good one.
If you're really interested in doing it I'd get photoshop or download GIMP (that's the free one) and play around with it. Learn how to manipulate images, cut things out of images (ie, the players in an in-game pic) and create interesting backgrounds.
An involved tutorial for them is way, way beyond the scope of this message board, but if you google for photoshop tutorials you'll come up with tons of hits that can teach you basically anything you need to know.
edit: And for the different size pictures, that's just wierd if they were the same size on your computer. I don't know what you could do to accidentaly resize them.