Versions of Outlook prior to 2013 would allow you to create an HTML signature with references to local images (e.g., logo, etc.) that would be converted to embedded images when you sent the email message. As of Outlook 2013, the links to the images are not converted so, if the image is not accessible to the recipient (such as in a URL or shared file on an internal network), the recipient will only see a broken link reference.
This can be remedied by using the Outlook editor to create the signature with images but the editor doesn't always give you the HTML you would prefer. An alternative is to make the allowing registry entries to have Outlook embed images like it used to.
Value type: REG_DWORD
Value name; Send Pictures With Document