It is pretty simple to add an email signature in Salesforce, so all email sent out will have your signature at the bottom of the email. Click your avatar, select Settings then My Email Setting.
Adding Image and clickable links
However, many users would like to add the company logo or special image in their email signature, how to add this?
You can't simply copy and paste the image to the signature box. So, a little effort here, you need to upload the image to "somewhere" so it is available for the public, such as https://inspireplanner.com/wp-content/uploads/2020/06/SimplySfdc-logo.png
Or, you can upload the image to Files in Salesforce, here is the step
2. Open the image and click Public Link