1. Open your Gmail account and click on Settings as shown in image:
2. Then in General tab of Settings page scroll down and type the name and other information in Signature box and click on the image icon to add logo or an image to your signature as shown below:
3. Then the Add an image window will open and there you can upload an image from your computer or from Google Drive (My Drive) or from website URL, as shown below:
Your signature is now added with logo.