You need to do some work aroud it since fend hashes the logo file information.
1. If you already uploaded your logo delete it (log in as admin, and update company info)
2. create you own logo file (best in png format) and save it in public\assets\themes\default\images\layout\logo.png, overwriting original file.
3. edit application\layouts\website.php. On line 77 (according to my editor anyway) replace <td style="width:60px"> with the pixel width value of your logo image eg. <td style="width:100px">
4. edit public\assets\themes\default\stylesheets\general\construction.css. Find definition of #logodiv and update the heigh& width properties to match you logo image.