After dragging the picture to the timeline, have a look at the preview window, On the just added picture, there are 8 handles at the border of the picture. 4 in the corners, 4 in the middle of the sides. You can drag these handles to enlarge the picture. Press the SHIFT butoon while dragging the handles to maintain aspect ratio. You can change the zoom of the preview window from "fit" to an other value to see the handles better.
Good luck
Hatti
Win 10 64, i7-4790k, 32GB Ram, 256 GB SSD, SATA 2TB, SATA 4TB, NVidia GTX1080 8GB, LG 34" 4K Wide, AOC 24" 1080