It's an interesting question, but it's VERY difficult to answer - at least for non-exclusives. It involves keeping track of pictures across all agencies. So at present, I do not know exactly - although I have a feeling that a minority of pictures accounts for the large majority of sales.

To properly answer this as an independent, you'd need Lookstat with Shutterstock.

My sales have always been from a wide variety across my whole portfolio. 

In line with several of the above posters, I estimate about 8% of my pictures bring in more than 50% of the bacon.  This is slightly skewed by a single image which had 2 ELs.  Ignoring those ELs, probably around 10-12% of images bring 50% of royalties.

I should study those top images more carefully and try to reproduce their magic formula (if any).  It would save a lot of effort to quit wasting upload quotas, processing time, etc. on the approximately 2/3 of my portfolio that have zero downloads.

Since starting out I've found that one of my images accounts for over 50% of my total earnings. Do you find a similar trend?

Yep. Been at it 2.5 months. My FIRST shot taken and ULed rules my roost. Thank Gawd for that one.

OX
...lucky shooter

I should study those top images more carefully and try to reproduce their magic formula (if any).  It would save a lot of effort to quit wasting upload quotas, processing time, etc. on the approximately 2/3 of my portfolio that have zero downloads.

I tried that with my "rain maker"... so far it has only been a nice idea that hasn't worked.

OX
...trying to beat the system

   If it were my site I'd give automatic feedback to each contributor regarding good performers. Why not a "Your Best Selling Images" column? Or automatic responses to a hot selling image: "Have more like this? This is your number 5 best selling image."

DT has it, right on your profile front page. Just like IS.

gaja:
The top of the SS list are certainly not the "best performers", only the good performers that have been there the longest. The SS reports you mention are only the accumulation of downloads, with no way to measure the success of an image relative to time on line. Most of the top of my list there have been on line for 6 years whereas an image I uploaded last month can only be found near the bottom by scrolling down through dozens of pages. Even then there is no way to track VELOCITY of sales such as is possible with iStocks "Dls per month" column. A simple programming change at SS to add a downloads per month calculation would be a great tool, especially for those with 500+ images.
   As an example, my best selling image on iStock is only there due to an accident. Using DT's sales results (much better than SS's) I saw a large number of sales of that image and realized that the same image was refused by iStock when originally uploaded. So I resubmitted the image to iStock three months ago and it is now my most frequently downloaded image there by far. And because it was easy to track the results on iStock using the DLs/month column I made sure to upload it to other sites I had overlooked. Without that tracking ability I would have missed out on hundreds of dollars in sales at iStock and others.
 

closed so soon?

Sorry! My first poll (amateur!). I don't think I can reopen the voting without stating a new poll.

Rob