Some people have had issues importing various movie formats into iPhoto. Even .MOV QuickTime movies sometimes don’t play correctly in iMovie. .MTS files from Sony cameras simply will not work – they need to be converted. If you have the choice of a conversion format, use the H.264 format which may have an extension of .mp4. This works fine and may be the very format that iPhoto itself uses.
You can see the format that iPhoto uses by choosing a video that has already been imported (and plays correctly). Then click File -> Reveal in Finder -> Original File. Then right-click and choose Get Info. Under “More Info” there is a Codecs listing which shows the codec of the video.
A good converter is Xilisoft HD Video Converter.