A red X has appeared in the upper right side of the highlight frame of one one of the pictures in my slide show. I don't know what it means, how it got there, or how to get rid of it. I can find nothing about in iPhoto "Help." What gives?

It means the photos is tagged as Hidden.

how it got there, or how to get rid of it. I can find nothing about in iPhoto "Help." What gives?

You hide or unhide a photo by selecting it and pressing the key combination ⌘L (or use the command "Photos > Hide/Unhide Photo")

That it is hidden in Photos and all Photos view

Not from albums

LN

What does "hidden" mean?

Hidden Photos are only hidden in the Library views (Moments, Collections, Years) - the views you can access from the topmost "Photos" icon in the sidebar. This way you can reduce the number of photos that are shown in the Moments, without deleting the photos from the library. It is not meant to hide photos that must be kept secret.

From the Photos Help: You can hide selected photos and show them only when you want to see them. Hiding a photo can be useful if you don’t want to use it, but you’re not sure you want to delete it.

For example: I have hidden all photos, and the Moments are showing this - a completely empty library.

But any album in the Album section of the sidebar, including the "All Photos" album is showing all photos it uses. With "All Photos" selected I am still seeing the complete test library.