JPEG poor quality

First I tried deactivating all plugins and going back to default theme, but uploading high quality jpeg always give me a blurred image, and downloading it give me a blurred one too.
See the comparison here.
I tried all possible browsers and the problem is still there. But opening or downloading the same blurred image using FTP give me the high quality result.
I am asking this here because I’m using wordpress and it happens to my website and not to other images in the internet (in case you think about a low connection or computer reason). This problem is not a browser neither a plugin or theme related as I mentioned above.
The problem occurs only to JPEG. The PNG format is always clean and safe.
Any suggestions? Thanks.