Image quality has more to do with;
How sharp is the lens,
How true are the colors,
How good is the exposure,
The talent of the photographer.
Megapixels relate to the amount of detail in the image.
A small photo does not need as much detail.
The quality settings ("normal" "fine" "superfine") shown in the chart are about the amount of compression that is applied to a .jpg image.
For the best quality, set it to superfine and leave it there. The only reason to use one of the other settings is to crowd more photos onto a memory card.
Leave it set to superfine and buy more memory cards.
I ran across a site that has a good write-up about the mathmatics of photography.
See if it helps:
I'm struggling to understand how image size and image quality relate. I have a 7.1mb point and shoot. I want to produce the highest quality photos that will be less than 4 x 6".
Why are there sizes of resolution? Is a "Large" at 3072 x 2304 actually larger(in height and width) on a computer moniter than a "Small" at 640 x 480? Or are all the images all the same size(in height and width) raw from the camera and the different settings refer to how much data is stored in each file?
What does quality mean? Is it a way to further control file size within each resolution size mode?
If I want to create a high quality 2"x3" image, I would use the "Large" resolution at "Superfine" quality to benefit from all 7.1mb? If I want to create a high quality 8"x10", I would use the same, right?
The chart I'm referencing is a little more than half way down the following page...