Did you use Export > Quicktime in iPhoto and just set a duration ?
If so these quicktimes you created actually have each image on a seperate track in the quicktime file with a crossfade trk that has info on when QT mixes from one to another. It must not be a compatible format for FCP to have a QT with a cross fade track.
All I can suggest is loading the qt from iphoto into QT player then using Export to flatten the movie into one single track. But!.. you will loose the cross fades doing so. Imprt FCP.. razor them to cut up then insert dissolves..
Or just import all the stills into FCP and do dissolves. This way you can vary the duration also as it seems iphoto is stuck to a single duration for all dissolves.
P