You can if you must, inspect element (using chrome) find the path of logo name and folder in server, go into cpanel (resources/images/..etc ) and literally replace that file with one named exactly the same name and size wise,(eg 30px - 30px and f.jpg or whatever) delete and replace, make sure you match the dimensions and name as I believe (in my experience) size specific for the top header and you should be ok...
I was going to say the same thing. I have been working on our mobile site with AbanteCart for a month and half and the easiest way I found was to inspect the elements with the browsers tools or Firebug and it would show me the name and the path to the file. I would then open the file in Photoshop.
Next I would see it's dimensions and create a blank file with the same dimensions and then when I was satisfied with the new graphic I would save exactly with the same name and format and then upload to the server.
I overwrite the existing file (backup original with you wish) and go back to the site and refresh and done deal. To me is easier and much faster.
Also since I plan to install for our company AbanteCart cart on a few sites having the same look Admin gets confusing so at the moment I am actively inspecting elements and will change the colors and logos in each admin so none of us are ever confused. I did this with our existing admin systems and it helped a lot.
Hope this helps and attached is a sample of what I did for Mobile.