If you enabled the image upload option in your testimonial form, then here you can set the default image.

Enter "0" to "not display any image if someone do not upload his/her image".
Enter "1" to use "Gray User image if there is no image".
Enter "2" to use "Black User (Male) image if there is no image".
Enter "3" to use "Black User (Female) image if there is no image".