Here's a demo on using photoshop to help you detect rare die varieties—even from analyzing small coin photos on
ebay! As an example, I'll go through the process of my recent find: an 1870
IHC Snow-8, which bring the total known copies of this rarity to 10 examples.

I first heard of this variety through Fivaz/Stanton's "Cherrypicker's Guide", and Rick Snow's very well-documented Attribution Guides. With that knowledge in hand, I went looking for this admittedly rare coin on
ebay. Eventually, I came across this coin:

I guess something was there that caught my eye, so I downloaded this image, brought it into photoshop, and by resampling to a higher resolution and adjusting the Levels and Brightness/Highlight, I was able to resolve a few more details below, which I compared to Snow's attribution guide.
Coin in question

Comparing to Snow's attribution guide:

There were some interesting similarities, notably the marks in the denticles under 0, the die crack running under 70, and a few other (admittedly subtle!) similarities on the reverse. What's more, the position of the date was identical to the S-8 variety in relation to the portrait and denticles: a good starting point for detecting any varieties. Upon receiving this coin, I was able to confirm with other helpful collectors this indeed is a new copy of the S-8. Click on the photo below to see a high-resolution example.
Obverse detail:
Reverse, showing doubling of both Ns in ONE CENT: 
With a little work and application of imaging software, it's possible to cherry-pick coins that go otherwise unnoticed.