All images in an email will be viewed as an attachment and added as such. It wouldn't matter what the attachment is, if this is a signature or an actual image, it will still be looked on as an attachment and as you have this linked to your Attachment Collection on your Incident it will create one Attachment collection per attachment in the email.
We added features in 7.8 and 7.8.1 to allow multiple attachments to be added to a single attribute rather than as separate instances of the Attachment collection. Furthermore, if you map your inbound email to an attachment attribute AND the attachment collection then it will attach the largest item (under the limit of 3.8MB) to the single attribute and the rest to the attachment collection. In essence, this means that the single attachment will be the screenshot or document in the email and the signature images will be stored out of the way. Have a look in the documentation for more information about this.
The documentation link does't seem to work.
The link above had some extra characters. Here is the long version. =)