There are a couple of potential causes of this issue. There is no way to adjust the picture brightness/contrast directly through the Slingbox, as that is not within the functionality of the device. The Slingbox simply streams the video content it receives. Is this issue happening on multiple devices that you use to access and view your Slingbox? If you have tested from multiple devices (i.e. PC/Mac, mobile device) and the picture quality is the same on each, then there is most likely something going on at the home location. It could possibly be the cabling between your cable box and the Slingbox, or a picture quality setting on your cable box itself.
If you are only experiencing the issue on one device, then it might be just a brightness setting on that specific piece of equipment. I know from personal experience that when the latest software update for Android was available, the auto-brightness settings on my tablet were too dark, and I had to go through the settings menu and manually adjust them.
Here are some additional resources that you might find helpful:
- Slingbox.com - How to optimize streaming in SlingPlayer
- Slingbox.com - SlingPlayer products for mobile devices get poor video quality
Hope this information was useful!