This image illustrates the two extremes of quality. The images you will rate will fall somewhere in between. To rate images, you'll use a slider input at the bottom of the screen to tell us what you think of the quality of each image. That slider looks something like this:
If an image looks worse to you, you'll drag the slider toward the left. If an image sample looks better, you'll drag the slider to toward the right. Once you've settled on a rating, a submit button will appear. Click it to record your rating and proceed to the next sample.
You'll be asked to review 25 images, but if you get bored, simply leave at any time. If you're ready, adjust your display brightness (if necessary) and click the button to begin: