If I want to use an image as a signature, what size should it be? Or does the site simply shrink it to fit? Can I simply paste the image into the sig box or does it have to be somewhere online with a link to it? And is the same true if I just want to put an image into a comment or diary? Does it have to be online?

If they have to be online, where is a good place? I've not had a need for a such a site before.