Thank you RufusW.
Thank you FRANPAN for the adjectives idea for the Hidden Picture Game.
Here is my amateur how-to video for sending the images over the purple boxes to the back for the "Chase that Vocab!" PPT Game.
Right-click on the slide you're adding images to over the purple boxes, and choose the "reorder overlapping objects" option (not sure if it's the same option in Windows).
Then move the slides with the images on them to the back of all the other slides, but make sure these slides with images on them are all behind
the yellow background slide (I didn't move my slides behind the yellow background in my video).
For "how to send to back" for amending "The Hidden Picture Game" in PPT, the following helpful YouTube video shows how. https://www.youtube.com/watch?v=RC8894eOruQ
Or watch my other amateur how-to-video.