You will have to use URI encoded images in these signatures. So they are completely embedded into the HTML code, no need for images on any server. We are using this method for our company logo in our signature. As a side effect this signature also bypasses attachment restrictions - the logo will always be shown.

You're needlessly filling people's harddisks and make them look at pictures they're not interested in to see. Whever you send e-mail, use plain text and only attach pictures if the receiver explicitly agrees to receive the files.
Moreover, my server is very likely to reject your e-mails as spam.

Moreover, you should know the difference between spam filtering and virus protection, when you make such a statement...