It’s the default “special” border that appears when you use an img element with an a src attribute set to something that doesn’t exist (or no src at all).
A common workaround is to set the src to a blank.gif file:
<img class="logo" src="https://stackoverflow.com/questions/5743083/blank.gif" />
I have to point out that it (in this case) makes no sense to use an <img> with background-image. Just set the src attribute and forget about background-image.