Please Help picture over the top of picture and video

We have been using devinci resolve 15.0.1 and cannot seem to find the tool that allows us to lengthen the Picture as a whole with out going over the next picture. We are able to lengthen a certain picture, but by lengthening that picture it only shortens the one next to it. We were wondering if there was a setting that allows the video to be lengthened as a whole rather than just a single picture at a time. Another way to describe this problem is we have a series of photos and one of them is too short or too long we try to shorten it, it leaves a gap, and if we lengthen it, it cuts into the photo next to it rather then lengthening the entire video.