The Add logo button is at the left top in the page header
Upload your own logo or Select from Google Drive, Google Photos, or from a URL
Your logo appears at the top left of the page header
Note
If you create your own logo, be aware that the logo is 40 pixels tall.
Set Logo and Navigation Background Colour
Click on the Add/Edit logo button at top left of the header, and choose Transparent, White or Black for the logo (and top navigation) background colour
Add Alt text to Logo - new Google Sites
Hove your mouse over the site logo
Click the three-dot More options button that appears on the right of Edit logo and choose the Add alt text option
Enter the text in the Descriptionbox and then use the Apply button