I know there is a plugin for Firefox but is there any other way to create a signature in Gmail that includes an image?
Google recently introduced their "rich text signatures" feature for Gmail users (including Apps users): Official Gmail Blog.
From your inbox screen, click on "Settings" in the upper-right corner. Scroll down to the signature box, which is now a rich text editor just like the regular email editor. You should be able to add images, colorful text, and all kinds of things that are totally unnecessary for a wholly-textual experience like email.