Animoto is what I use all the time. I have a pro account so all the licensing is sorted, though that does not stop the wolves prowling youtube trying it on. I believe you can also pay per video though if you just one the odd one, though check the licensing suits your needs.
I could be missing the point here ( apologies if I have ) but could the slide show ( assuming its digital images ) be done with Powerpoint and then music added? you can select how long each image is displayed and how the images change.