Make sure you in your Google Email inbox, then click on the settings (gear icon) and then see all settings.
Under the General tab, scroll down until you see the signature editing area. Add, delete, or modify any text to your signature area. When you are ready to add the logo, place the cursor where you want it and then click on add image icon
Add an image window will open so that you can enter the image URL, enter the image URL in the area to the left of "image URL." If done correctly, then logo will show in the preview area. Click OK to continue
Now that you are back to the signature ediiting area, your logo can be resized by click on the image once and selecting the image size: small, medium, large, etc.
Scroll down to the very end of the general tab area and click on save settings to save your signature settings and changes.