How to measure the quality (or difference in quality) of different images?
It all depends on what you mean by "quality".  If it is visual quality as perceived by people, then the best thing to do is to ask multiple people to compare pairs of images to see which ones they like better.

Since you have mentioned a vision application, then it must have some performance metrics for whatever it does, which would be its measure of quality.

If by quality you mean how blurry or noisy the image is, then you might compute the Fourier transform to see whether the image contains high frequency components to see how blurry it is, or compute its entropy to see how noisy it is.