If the images within your email signature are being saved onto the file history as separate attachments, it will be because the signature images are larger than 400kb. The email agent will not process images (and therefore add them to the file history) if they are below this size. This limit cannot be changed.
The two options would be to either turn off splitting attachments from within the email agent which is a firmwide change and it would stop processing all email attachments for all users.
If you would like further information on how to do this, please raise a new case online and reference the title of this article.
Alternatively, you can shrink the image that is being used in the signature so it is below the 400kb limit which would stop this from occurring.