Navigate to the site builder either in a funnel or a website. This will be in Sites, and either Funnels or Websites
Choose the Image element from the list of elements on the left-hand side or select an existing image element from your page.
Under the element settings (on the right-hand side) toggle image optimization.
Please Note: There is also a toggle for this in funnel settings, keep in mind that the element in the builder will override these settings. You can enable or disable this by default though.
