The photos you've used in your video may have different sizes and resolutions, and that may cause unexpected results when your videos are done. To fix this, do the following steps:
After you've selected your photos, tap .
From the Edit order & focus screen, tap a photo then tap to adjust the photo’s display area.
Tap and drag the photo to adjust its focal point. You can also spread apart/bring together your two fingers to zoom in/zoom out your photos.
If you want to further edit your photos, use the tools on the toolbar.
After finishing the desired edits, tap to save the changes.