I used PhotoStory 9 Deluxe and most of my photos are from 3 to 6 MB each, the program is fine. The real issue is whether the computer has the resources for that playback.
A presentation HD format is 1920 pixels x 1080 pixels at 120 dpi. same presentation in SD (standard video) is 640 pixels x 480 pixels at 72 dpi.
New digital camera give a format about 5000 pixels because we need to print photos ( 5 megs) you can use in presentation as in and the software reduce the size automatically in theory no problem about size. If you use photos for a web you need to reduce the size because you need that the web page display very fast on the computer. If each photo on aweb page use 5 minutes to be seen chance that nobody view your page. Pro users like to work on photos ans adapt the size according to the end use.
You can view my training video in the tutorial photo section,