I'm not sure what you mean when you say "the video clips are being clipped off". They should not be clipped off unless possibly you are using a transition of more than 2 seconds duration and you had to trim back the clip to get more residual media for the transition to overlap onto. (See below for an explanation.)
In iMovie 10 transitions hook on to the invisible residual media that is left over when you trim a video (not photo) clip. So, when you remove (trim) part of a video clip the trimmed part is not really removed but just becomes invisible in the timeline. If you try to change a transition length to make it longer than the default 2 seconds you often will get a message that there is not enough media available to change the length of the transition. This means that there is not enough residual media for the transition to overlap onto. Although it seems counter intuitive, the way to get more residual media for the transition is to trim back, not lengthen, the video clips on either side of the transition. That frees up more residual media for the transition to hook on to. Of course, trimming back the clip means that the trimmed portion will not fully display before the transition starts, and possibly that is what you mean by the clips being clipped off.
With photo clips it is different. Photos are not trimmed like a video clip but rather are shortened or lengthened in duration. Unlike a video clip, when you shorten the duration of a photo you would not be eliminating any part of the photo itself -- just displaying it for a shorter time. Since you are not eliminating any media there is no residual media created when you, for example, shorten the duration of a photo from 6 seconds to 3 seconds. As long as the photo has sufficient duration to accommodate the length of the transition you can create any length of transition that you want. If the photo is not of sufficient duration then you would increase the duration to accommodate the length of the transition. This is the opposite of video clips where you would trim back the clip instead of increasing it in order to create more residual media.
-- Rich
