Site image is used every time someone shares the URL of your blog's homepage. For example, on Facebook it looks like this:
To upload a site image, from the left menu, go to Online site > Preferences. Alternatively, you can type "Site image" in the search bar on the top of the page and click on Preferences.
You will be directed to the Preferences page, select the Hompage tab from the top panel.
Scroll down to Site image, and upload your images. The recommended size is 1200 x 630 pixels.
After finishing all your changes, click the Save button on the top right of the page.