Using Adobe Premiere Pro CS4, a transition between 2 still images can sometimes stop working after the sequence is rendered. The symptom for this problem is that the transition seems to "hard cut" between the images instead of displaying the transition's designed behavior.
The workaround is to add an Opacity keyframe to either of the clips involved in the transition. You don't have to change its value; just leave it at 100%. It also doesn't matter what frame you use to set the keyframe.