This article explains how to display an image for a testimonial that originally featured a video.
Why you cannot display both an image and a video
A testimonial can only feature a video or an image. You cannot display both media types on a single testimonial.
How to display an image instead of a video
You can re-import the testimonial as a text-based one. Link this new testimonial to the same customer.
This creates two separate reviews from the same customer. One review will contain the video. The other review will allow you to add an image.
You can manually import the testimonial. Visit the manual import page here to add the text-based testimonial.