Hi, 

I had a similar issue with the logo. What I found was that if I made sure that the dimensions of the image were identical to the dimensions in the navigation.less file, the image showed. Changing those to anything other than 1:1 correspondence, and you're in trouble. E.g.,

.navigation-inner {

.logo {
margin-top: 12px;
margin-left: 25px;
float: left;
width: 45px; /*change this to the same width as your logo image*/
height: 17px; /*change this to the same height as your logo image*/
       background-image: url(assets/logo2.png);
}

If you're not sure how to find out the dimensions of your image, (for MacOS) just click right and choose 'Get Info' and you'll find the dimensions.