Hey there, thought I would share my experiences here as I spent many hours trying to make this work.
When you get a new LED TV, the first thing you should always do is calibrate the picture settings as generally they are setting wrong and the contrast and white balance are all too high.
There are LOADS of sites that offer the best settings for certain situations, of course only you can deide what you like, and to do it properly I would suggest downloading some of the burnable images that has test patterns and tutorials for getting the look just right for your situation.
However on the new Samsung @ Internet TVs you will find that when you set the default picture settings for your TV Input and your HDMI Input, these go out the window when you open a Widget from the @ Internet App List. In my situation I run the Samsung Plex client. So everytime I open this it defaults back to default settings and loses the tweaked image I have configured.
After much asking around and calls logged to Samsung I have discovered the best way to do this (at least for Plex) is to open the Widget in question, and then press the 'Tools' button on your remote.
In my situation I had to wait until video was actually being streamed from my server to the TV, upon pressing Tools I got the basic set of picture options to apply to the Widget video stream. I was able to set the same 'calibrated' settings I had used elsewhere.
Hope this helps others who are struggling to find these settings which aren't documented anywhere that I could find :)