In order to just replace the step's image while retaining the rest of the elements (voiceover, elements, etc.):

1. Click 'Edit Video'
2. Select a video step where you want to change an image.

3. Click 'Crop'

4. Click 'Add Photo'

5. Pick an image from your library

6. Adjust the size of the inserted image

7. Click 'Confirm' to apply a change.
​