When I import a picture to my project it places a random photo previously used in the project. It happens when I post either a group or single photos.

I am trying to complete a photo slide show using iMovie. I have imported several groups of files and added music to the show. Now, when I am importing the final set of photos (either single or as a group), it is pulling a random photo from previously in the show and inserting it in place of the photos I am trying to import. I can't figure out why the problem has suddenly started occurring. I am following same process I have used throughout the project. Any ideas greatly appreciated.