Hi,
when you go to Testimonial in your Dashboard, click on Add new. There you can set all for your testimonial. When you edit some of yours testimonial, set all details for Client data and in the right bottom corner you have Featured Image. Click Add new feature image and upload whatever image you want. After that edit your page, set there element Testimonial slider and you will see Testimonial image when you view that page.
If you can’t do this, please send us your WordPress credentials so we can take a better look and help you.
See screenshot in attachment.
Kind regards