The Saturday Evening Post had many artists. The best known of those was Norman Rockwell. Use the site below for more information.
Norman Rockwell is probably who you mean.
The value depends on several factors. What the covers are about, when they were published, and the condition they are in. If a famous person is on the cover they could be worth more depending on the...
My guess from what I have read would be from 1821 - 1969. DB