Keynote version 8.3 how to alter image opacity.. only shows gallery/arrange and not style tab?
To fade an image in Keynote, use the Dissolve animation.

Steps:


Select the desired image
Go to Animations
Select Build Out
Choose the Dissolve animation
Set the desired timing and whether to animate on click or with another action


For more information on animations, check out the Animate Objects in the Help menu.