I have tried to make comparisons several times, with different games. If there is any difference in picture quality, is unable to notice unless you have them side by side. As far as running smoother and shorter loads...I can't tell any difference, between testing 5 games. So, I just run from the disk.
You're right. Installing normally doesn't change anything for image quality.
It only helps with loading times, but can help with texture streaming, too. So it can indirectly affect how the game looks by for example reducing texture pop-ins.
But most games don't really benefit visual, only better loading times.
And apparently there is a bug with Skyrim (supposed to be fixed with the day 1 patch), where installed the game doesn't always load the normal textures and objects appear low res.
That's what this thread is about.