Exclaimer
Exclaimer is the recognized global market leader in on-premises and cloud-based email signature software and solutions.
To add imagery to an HTML email signature, you have two options available to you. You can either add a logo/image by embedding it directly into the signature or hosting it externally and linking to it.
The vast majority of email clients display logos/images in an email signature without any major issues, so there might not appear to be much difference between using the two formats. However, there are benefits to choosing one method over the other.