Hey Elena,
Thanks for reaching out and I hope you're having a great day so far.
Diving straight into your questions, using a Rich Text editor, the easiest way to create "space" on either side of your emails is to build it into your email layout. This help document will take you through the steps on how to create and apply a layout to your email: https://customer.io/docs/3-layouts-and-customerio/.
With regards to the email signature image, you have 2 options:
1) Build it into the email layout as well so that it applies automatically each time you use this layout
2) Use the insert image button to manually add the image to your email
Hope this helps to clear things up but if you have any further questions about this or if I can help with anything else, please don't hesitate to reach out!
We'd be happy to help. :)