Transitions Don.t Work on .png Files?

Does anyone know why transitions don't seem to work on a .png file (photograph) when viewed in a browser? I tried to have a .png photograph "wipe" in (and it looks fine in native Lectora), but, when I published to HTML (viewed in browser), the photograph simply appears, rather than wiping. The transition for the text box that's set up to wipe in with the graphic works just fine -- it's just the photograph that won't use any of the "fancy" transition types. (I tried a few other transition types, just to make sure the problem wasn't limited to the wipe transition, but none of the other types I tried worked either.) I also tried the transitions with other graphic types, and those work just fine.Unless someone has an idea of why .png files shouldn't be able to transition in, I guess I'll report it as a bug!LauraP.S. I'm using Lectora 2008, SP2, with IE6 (no choice there).

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