how can I attach the signature logo on sending mail?
The FileMaker Send Mail script step don't use the signature so you'll need to specify it manually or add a signature made in FileMaker to your email. You'll need to store the logo in a FileMaker table and export the field contents and then attach it to the Send Mail step.
Here's an old example file in FileMaker 7 but it will work fine in FileMaker 16 if you convert it.
Retrieving data ...