Embedding images in sig in Thunderbird

This is for my boss actually - he has a series of logos that he wants to include in his signature, and wants to know how to do this without them being sent as attachments.

I've had a quick look around but haven't been able to find any clear instructions.

Thanks,

SG

 

Just to clarify, the logo images aren't appearing as attachments when I check them through Gmail, but are (as MIME and PNG) when I check them in Mail. Any way of avoiding this altogether?