I found the perfect way to make a portfolio more efficient. You have to make this equation.

( Average number of megabites of all files /  the average mpx size that were taken all the pictures (x) by the crop ratio of the camera)
                                                               (X)
(Total online files / by the number who have sold 1 time (x) number of files that make 10% of your incomes (starting by the best sellers and counting how many to the time it make 10%))


Here's my result of my Istock portfolio : (5.6 / 10.6=0.53) (0.53 x 1.5= 0.80)  =0.80

                                                                              (105 / 51= 2.0) (2.0 x 3= 6) =6

 Final operation ( 0.80 x 6= 4.8 )   =4.8

So that's giving you the exact number of megabites for a picture to have that will make you rich! My best seller on Istock is the nearest whit 4.6mb Ohhh!!! That's scary

bwahaha....  I agree with le_cyclope....