A good application that can be used to adjust webcam and video quality is Video4Linux Control Centre
This program allows you to change various parameters such as brightness, sharpness and contrast to hopefully improve the quality of your webcam/video capture device's picture quality.
To see the effects of any adjustments you are making you can use Skype. Enter the Skype options menu and select Video Devices and click the Test button. With this window open launch Video4Linux Control Centre and any changes you make to the settings can be seen to take effect in the preview window of Skype (You can double click the preview window to view this full screen). Or you could use a dedicated webcam application such as Cheese to view your video device's picture as you make changes.