Picture in content is not really sharp

Hi!
My first picture on my site (http://www.geri.at) is slightly unsharp.

Don*t mean the dof (depth of field) of the photo – every picture is not as sharp as it should be.
Clicking the picture it is sharp agaain, even it’s shrinked to the displaying dimensions.

Any clue?

Thn!
Greets
Geri