How do Credit packs work?

Hi,

I am considering purchasing some images but am wondering how the 'Credit packs' work?  I cannot seem to find any helpful/appropriate advise on the main web pages.

If I want to purchase a particular image e.g 176880033, how can I tell how many credits it is worth? Are some images more expensive than others? Or is that not how it works? 

Tyia

John