I disagree that one transition is always best with photos. It gets repetitious. But, the transitions you use should NOT distract from the photos. Rather, they should enhance the photos. There are plenty of tools, such as slowly zooming toward the focal point of a photo, that make a presentation a bit more interesting.
edit: a powerpoint presentation where the presentation is there to enhance what the speaker is saying, not distract from the speaker - one transition, definitely. But, if you're trying to keep people focused on the photos, then more than one transition. That's not to say that 90% of the transitions are appropriate. 90% of them, even when used as the only transition, make it look like a kid with a new toy.