I don't think you can judge a horse by photo -- particularly with race horses you want to see how they move and their character.
The TOBA has an informative series on conformation.
http://www.toba.org/owner-education/conformation.aspx
After awhile most people form preferences and have ideals in the kind of runner they are looking for. For example, I don't like the light-frameness of Songandaprayers and am particular about necks, pasterns and feet. You have to approach a horse with a goal in mind too -- are you looking for a win early 2 yo, a Classic horse, a turf horse, etc.