lorde It shouldn't break your site if you enter all the values. There are three values you need, the logo's URL (http://example.com/mylogo.jpg), the logo width and the logo height in pixels (you can usually find this by right clicking on an image and viewing the image info/properties).
So once you're done, your code should look like this:
h1.Header-title a {
text-indent: -1000000px;
overflow: hidden;
display: block;
width: 25px;
height: 25px;
background: url("http://example.com/mylogo.jpg") no-repeat;
}
Note the semicolons and quotes where appropriate.
