Other than subjectively looking at the image to judge its quality, the only facts I have are its width and height in pixels and its file size. How do I use these facts to be reasonably sure that the image I'm buying is of a reasonable quality?
Is it true that, with a sweeping generalisation, that the bigger the total pixels to file size ratio is gives poor quality and the smaller good quality?
If so, what would you say is the maximum total pixels to file size ratio I should use to be assured of good quality when viewing the image on a website?
I understand this is a complex, subjective, subject but it would be good to have some sort of guidelines.