HTML Overlays have recently been renamed to ‘Layer Slides’, but the same functionality exists.
1. Click ‘Add Slide’
2. On the left of the Media Library you should see a list of Pro slide types (YouTube, Vimeo, Post Feed & Layer Slide)
3. Select ‘Layer Slide’
4. Select the background image to use for the slide
5. Click ‘Add to slider’
6. Click ‘Edit Source’ to reveal the HTML text area