A picture is worth a thousand words.
So why not use them in reactions to increase their effect? Now you can!

When creating a Lightbox, Notification or Sidebar reaction, you can now add an image: as an element or as the background for the reaction.

Select the layer type you'd like to use - Lightbox, Notification or Sidebar.
When creating the card, activate the image option:

You can then upload your image and select whether to use it as an element or as the background for the reaction (note - the sidebar only allows for the element option):

You will see a preview of what the reaction will look like as you create it:

Save the card and finish creating your reaction!