Thank you the fast response. Basically, I took these steps:

1. admin->tools->Testimonials Manager
2. click on a testimonial that has an image added previously
3. click Edit
4. click Update w/o editing anything.

That's all. The image is now missing. I checked the image folder specially created for testimonial and found the image file still there.

Btw, I am doing this in my local PC though. Thanks for any advice.