It might help to review how transitions work.
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 on to. Although it seems counter intuitive, the way to get more media for the transition is to trim back, not lengthen, the video clips on either side of the transition. That frees up more resifdual media for the transition to hook on to.
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
