Give it more depth and detail by using more shadow and highlights (photoshop tools; burn and dodge). I mean, on a real picture that size the details of hair becomes less also. Just the disadvantage of smaller images... same as skin. On a large render you will see all the tiny details and if you shrink it down those details will be lost. If you paint hair on a smaller image the hair becomes too thick and will look more like rope to be honest. Really, using darker parts for shadow and lighter for highlights will make a huge difference. When strains of hair fall over another part of the hair it will create a shadow, no matter what color hair you paint. Hope that will help you a bit...