Put png over background image

Yes, it is possible to overlay several images onto a background image. As léonie points out you would need to create the composite in an app like the Preview app on your make, and then import and overlay the created composite onto the background photo. For example:

Make a duplicate of a random image on your desktop and then open the duplicate in Preview. Do an Edit/Select All, Edit/Delete, to create a blank transparency screen. Then open any other photo and do an Edit/Copy, Edit/Paste of the photo into the blank screen that you created. You can adjust the size and position of the photo. Repeat with other photos to create your composite. Then save and import the composite into imovie and overlay it onto your background photo. The effect will be like the example above. You can overlay it as a Picture in Picture so that you can adjust the composite's position and size on the background photo.

-- Rich

Photos does not support graphics composition.

You could create the image with the individual images superimposed in the background image with the Preview.app (or any graphic editor you own), then add this photo to your slideshow.