Depends what you have as your sig.
I feel a banner image is fine, but the ones that have a scrollbar to see it all are too big.
It does sometimes get ridiculous when the images are very large (file size or resolution) so please bear that in mind.
Oh, and Dark Blade - if you go to an image hosting site such as photobucket, you will need to upload your image to the site, which will be explained at the site.
You will then normally be given the code to embed it.