How to Display An Image Based on Field Validation Results?

How to Display An Image Based on Field Validation Results?

Good Afternoon,
 
I have a lengthy form that users must complete.  Inside the form, I want to place an image of a check mark if the field has been filled in, and an image of a red X if it's not.  How would I structure the script to evaluate a given field and make either image visible based on the evaluation results?
 
Thanks,
Sid