Hi Jan,
It depends on how you want to do the light effect. Do you have a link to an example with the light effect so I can make a suggestion on how to implement that in Google Web Designer?
In Google Web Designer, you can use the hover event to set an action. For example, you can set an event on mouseover of the page to show another image that was hidden.
In the CSS panel, hide the element you want to hide using visiblity: hidden.
Click on the + button in the Events panel on the right hand side.
Select the page for the target
Select Mouse > mouseover for event
Then you can select the action you want, for example, CSS > set styles to turn an element visibility to visible.
Hope it helps.
San
Google Web Designer team