Image blurred in post

Hey guys,
I hope you can help me here. This is my Problem:

I want to insert a graphic into a post and the original graphic is 590x300px. I click on «Add image», then I chose the file and click «fullsize» and «Insert into post». To get the image real fullsize I click at the edit button in the left-top corner of the image and go to «advanced settings». Then I chose «Original size» and click «Update».

When the image is in the post it has a bad quality, it looks blurred. If I go directly to the original image with my browser, the image looks fine. Why does the graphic looks so blurred in wordpress and how can I fix it.

I also tried to upload the image via FTP and insert the image, but it´s the same.

I hope you can help me.

cheers,
bneum