images lose quality using hover box

Im using hover box in my web, and when i set the images these lose more quality.
why?

e.g:

Original: http://genomapc.com/wp-content/uploads/2014/07/genomapc_oferta1.png
In Hover box: http://i.imgur.com/XXeTC5m.png

any solution? what resolution most they have to not lose it?
thanks

sorry for my english xD!

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