Hi again Staph.
As compusolver said, Flash is an incredibly powerful program. Something like a slideshow would be the very tip of the iceberg as far as it's power. I've seen sites where flash was used to link to a live webcam, and even allow users to trigger events happening on the camera. Pretty cool.
As far as what that company used, they could very well be using video editing software, even though they use still images. If you create a slideshow and you want to add music to it, you will still need to output to a video format, whether it's on DVD, VHS, or whatever, if you want others to enjoy it. that means at some point, your still image slide show turns into a 29.97 frame per second video. So why not take care of everything in one program?
If you buy, for example, Powerpoint, you're going to have to buy another program to turn that into a video which you can put onto DVD. With video software, it's all in one hit.
In Sony's Vegas (which I reccomend to you because it's fairly easy to learn), you simply take your .jpg files and drop them onto the timeline, and boom. Your still image is there. You can also add multiple layers of video, which means you can do text overlays, or transitions that you couldn't get elsewhere. Plus, and here's the thing to really remember: you can add video clips to your presentations by using a video editor! Let's say that one of your clients wants you to add a clip of her daughter's first steps into her wedding slideshow. With virtually every slide show software out there, inserting video is painful (one big exception is MediaShout, which does quite well at this). But with a video editing program, you can not only add it easily, but you can also add transitions and effects to it if need be.
Yes, you could buy slide show software to get your job done, but ultimately, you're going to need the extra power that you get with video editing software.