I use the photo app for my iPhone/iPad and if I want to get fancy, then iPhone on the iPad.
I prepare my master files in Photoshop and when I have a collection, I run Image Processor Pro II script to flatten and pare the files down to 2400 px high-quality jpeg files.
With a folder full of images, I drag it to iPhoto and then create an album in the order I wish.
When I sync my iPad and iPhone, it's just a matter of checking a box during syncing to get the album on your device.
Simple is good since I believe the images should stand out. Having bells and whistles, fancy transitions, etc., simply detracts from the visual appeal, IMO.
With the photo app on either device, the interface and presentation is simple. I use the iPad for one-to-one and small group presentations and then hook it up to my projector for larger groups and talks. Some people even like Keynote for this, but that's another program to download and learn...