Above, we have two screenshots of the game "Sonic the Hedgehog 2". The one of the left was made using a Genesis emulator and the other image is from the console unit itself. (Note: the colors and sizing of the screenshot made with the console are a little off. This is due to the equipment used in creating the screenshot.)
As you can see, the images are the same. In general, assuming the emulator used has been written correctly, there will be no difference in quality. In fact, due to the higher resolution of a computer monitor, the picture will be sharper on the emulator then on the actual console unit itself.
Knowing this advantage, some emulators actually enhance the image rendered by the emulator using interpolation techniques.