My bestselling stockphoto

I am a stockphoto contributor on different websites for three years now. Started with iStockPhoto but added a few more soon after. After a few months I noticed that the same image is a bestseller on all sites. What strikes me is that the margin against the number two best selling image is huge.  See the list:

• iStockPhoto     236%
• Dreamstime    450%
• Shutterstock   258%

Is this image so much more attractive then all others? I don't think so. 

© Photographer: Koshar | Agency: Dreamstime.com

The number two bestseller on Dreamstime is an aerial of the Bungle Bungles NP in Australia.

© Photographer: Koshar | Agency: Dreamstime.com

The number two seller on iStockphoto is  this portrait of a crocodile (Dreamstime link because iStock doesn't provide HTML include code) 

© Photographer: Koshar | Agency: Dreamstime.com

The only reason I can think of is the fact that the black crow is on a clear background so it is easier to extract.