Picture automatically reduces in post

Hello,

I am using wordpress and when I want to add a picture to my post, the picture size is automatically reduced. All my pictures have this problem.

Orginal Image Resolution

1920 × 1080

And how it looks like when I add image to post:

http://vpx.pl/i/2014/03/01/ttt.png

When I remove the class = “alignnone size-medium wp-image-37, then I can get it to normal size, but for it losing quality.