In the toolbar of the 'Visual' editor, there's an icon depicting a tree - that's for inserting images. That should allow you to do it, unless it's been disabled for signatures.
If it hasn't been disabled, pop the relevant image in your own web space, and use the HTML editor to put it in using an <img...> tag.
In either case, though, please ensure it's small and unobtrusive!
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Vince M Hudd - Soft Rock Software
(I only came here looking for fellow apiarists...)