The dark grey lane on the timeline is the primary storyline and is magnetic. If you place your images onto a lane above this, it is not magnetic so changes to duration will not result in what you want.
Depending on your workflow and the type out media you work with, iMovie may be a great choice. If you need more of the professional features in FCP, you can save yourself many, many hours of frustration by watching these free tutorials:
Steve at Ripple Training’s 60 minute FCP tutorial overview https://www.youtube.com/watch?v=HqUP7Zgeuck
Final Cut Bro getting started https://youtu.be/L3bZ-ETKZ8U
Izzy Video tutorials. Recorded several years ago so the UI looks different but the concepts are still correct. https://youtu.be/_jvyrnQpR1E?list=PLp5SAgbM8S87n1qvv34-PAU2zMHLlXyfb