[Noelia Muíño gonzález]

Hi!
Probably, your photo/video proportions is not the same as the composition you are working at. So, you can either do some of those solutions:
1. Change composition settings to the size of your photo/video. You can go on the up tab, on ‘Project’ > ‘Project settings’
2. Clicking on the image, you activate the manual selection to change its sie (make sure you use the arrow tool). Then, by clicking one of the corners resize it. If you want to keep the right proportion of the photo, you should press ‘alt’ key I think (or another one near XD), then it will become even bigger than your project size, but will keep its proportions.
3.If not, just resize manually to the sides, but your photo will have wrong proprtions.
Hope it helps 🙂