You will have to judge perceptual differences between two images. In other words, what do you feel is the distance between the two images. 
Example: 0 if they are the same, 100 if they are totally different. Indicate the degree of disimilarity by moving the slider, from left to right. Of course since the scene is the same in the two images, appreciation of differences might be difficult, but you should express your personal feelings.

The checkboxes on the left of the slider indicates which scene appears to be more natural.

The checkboxes on the right of the slider indicates which scene you like the best.

In each case, check one or the other button; both in case of equality. 

"Do not forgot to check you results, those numbers will be send to me by email. "