Here an image logo is placed in the top pallet using the pallet_top_title phplugins hook.
The image is 48px tall to match the height of the top pallet.

A little custom css was also needed to remove margin from a list item.

Here's the phplugins code:

function pallet_top_title() { 
  echo '

  <li><a href="http://yoursite.com">
       <img src="http://yoursite.com/path-to-image.png"/></a>
  </li>

  ';
  return false;
} // END

Of course, use the url to your own image.

Here's the custom css:

 .page__pallet__top > .content > ul > li {
    margin: 0;  
}