There could be various reasons why the image may be too dark or grainy. 

1. The image's overall lighting is uneven. This means that your ambient lighting may not offer the proper lighting that is needed for your photo. Make sure you use a ring light and even background to stabilize your ambient lighting. 

2. You have your device or DSLR camera at an angle. This means that you are not in plain frontal view; instead, you have the device either angled upward, downward or towards the side. This will cause your image to have shadows and will result in dark patches in your image. This will affect the coloration of your smile desing because you will see that parts of the smile are brighter or darker than others. 

3. You have too much zoom power on your device or DSLR camera. This will cause the quality of your image to be pixelated and will result in a grainy image. 

4. If you are using a DSLR camera in manual and your image is dark and grainy, you are operating the camera on incorrect exposure settings. Make sure that your ISO is at 400 to avoid noise/ pixelation in the image. Also, make sure you have adequate shutter speeds and f-stops to adjust the lighting in the image.  For further instruction on our recommendations when working with DSLR in manual, please refer to this article, How Do I Take the Perfect Frontal Facial Photo Using My DSLR camera?