Logo squished

By default, the logo gets sized to 80x80px, regardless of the size of the image you are using. 80×80 is the size of the default WordPress logo that you are replacing.
You can either resize the image to 80x80px, or add the following to the custom CSS box on the login logo options screen:

.login h1 a{
background-size:286px 80px;
width:286px;
}

The background size/widths are specific for the image it appears you are trying to use. If you decide to change the image later, or if someone else is having the same problem, replace the 286 with the width of the image, and 80 with the height.