Thanks to user spiritofcat who pointed out 6 images that were inverted, I was able to fix the images and now they look much better! The images impacted are listed below:
Track Workers
Norwegian Farmstead
Fast Friends
View Deck of Gunboat
View on Deck of Sidewheeler Hunchback
General Grant's Council of War
I also implemented a slide position indicator that shows you which slide number you are on. This makes it easier to tell where you are in the slideshow and it is displayed just below the projection screen.