You can change your option by using the radio buttons that appear at the top of the navigation bar. Higher quality images have the best clarity, but their file sizes are larger, resulting in a longer download time. Lower quality images are a little less clear, but they download faster. Also, even the lower quality images are excellent, and the difference between the high and low quality images may not be important to you.
Image size is self-explanatory.
Try both sizes and use the size you are happiest with. Larger-sized pictures also take a little longer to download over a slow Internet connection. It's quick, painless, and completely anonymous.