GroupwiseService.php Send Emails with Embedded Signatures Images

I am using the GroupwiseService.php project and I am able to send an Email with attachments and HTML body.
Now the only thing that is missing is be able to place an embedded image in the signature of the email.

The only way that I manage to do this was using a public URL of images and use the </img> tag with src pointing to a online image.

It works fine for hotmail and others but it dont work with GMAIL.

Is there any other way to do this like use the CID?

Can anyone please tell me how to do this and provide some code example?
I have searched on this forum but didnt find a thread that answer this.

Thanks in advance
Ricardo Braga