Let me help you out with that! It can be a little bit tricky since HTML/CSS don't have an easy way of creating hover effects for images, themselves.
What you will want to do is instead of adding an "Image," add a "Block" where you would like the image to be.
Make the block the size you would like your image to be, and then add the normal version of the image as a "Background"
Now you should see your image in the website design. To add the hover effect, with the "Block" selected, click on the pointer icon in the "Current Selector" field, and select "Hover."
Now just change the "Background" image for this "Hover" state to be the one you would like displayed when the user hovers over this block.
Like I said, it's a little complicated, but basically should be done with "Blocks" and "Background" images. Hope that helps!