Have you checked out FotoMagico? Its superb for quick slideshows with sound and slick transitions.
I also do a great deal of work with Keynote. It can do all you'll want and much much more, but the 'more' gets complex and time-consuming, but the end result can be spectacular. One caveat - Kn's sound handling is abysmal - it appears to use the main processor for sound and the graphics processor for images but they dont run in sync so your sound can vary wildly, from play to play, and from machine to machine, and worryingly vary even more when output to Quicktime. This is only important if you want exact sync of sound to action on screen otherwise its fine.
FotoMagico has much better sound implementation and can auto-sync slide sequences to match a soundtrack length, and it can 'almost' sync sound with image transitions BUT in reality it cant, despite FM advertizing that it can. In discussion with the FM dev team they have admitted to me that it cant consistently stay in sync. However that said for non-critical sync work its really very very good, but it does struggle with any complex image stuff - whereas Kn can actually do 'layers' of sorts with stuff all piling up on one slide to create the desired effect.
Both programmes have their strengths and I use Fm for some bits where I want to quickly match images to a soundtrack, pan and zoom on images, then output it as HD video and drop it back into Kn to do complex bits, and vice-versa - best of both worlds. www.john-macpherson-photography.com