There are two good ways to add images to outgoing emails in OE.
The first, setting stationery. The files need to be HTML files and are meant to be background watermark images (but can be 'solid') in so much that they permit themselves to be overwritten by text (either your own or someone else's in a reply to your email. You can set default stationery which is present when you opt to 'Create Mail'.
The second, and my preferred option, is to attach a 'Picture'. You could prepare a letterhead image (in any image format) and similarly a signature (which could be a scan of your own siggie) and store them together in an easily accessed folder. The images can't be overwritten by text but neither (I don't think) can they be set to appear in a newly created email by default.