I understand that you want to know how to control the picture brightness of your Slingbox SOLO.
If you are using the web base application through www.slingbox.com, you will not be able to. However, you can download the SlingPlayer software for Windows or Mac computers. The older SlingPlayer software version allows you to adjust the video settings. The following links will allow you to download and install the old SlingPlayer software version on your computer:
Please understand that you download and install these old versions of SlingPlayer at your own risk. We do not offer any support for these old versions, and they have not received any development or updates for some time. They may work for basic streaming if you choose to try them. Watch on Slingbox.com is our current, fully-updated, and fully supported viewer for Windows and Mac OS.
Hope this helps!