Hi Martin
I haven't done this myself so someone might come along with a better answer.
You can include a js file in the theme folder to make changes to the interface that can't be done with just css (you can see an example in the theme 'btnTopPurple' where there's a js file used to move the buttons from the footer bar to the header). You could use javascript/jquery in this file to insert a div or img tag into the header and then put the image in there. You would need to save your icon image in the theme folder and the file path to the image would have to use this format:
Code:
'<img src="' + x_themePath + 'btnTopPurple/icon.png">'
I hope this helps