Sometimes it may be desirable to replace the Joomla logo with your own customised logo in the administration panel. These can be easily done by manually changing some images in the administration template.
Using FTP go to the following directory
/administrator/templates/isis/images/joomla.png - Current dimension for Joomla 3.x is 256x53
/administrator/templates/isis/images/logo.png - Current dimension for Joomla 3.x is 143x30
/administrator/templates/isis/images/joomla-joomla.png - Current dimension for Joomla 3.x is 24x24
Please note keep the images the same dimensions or you will have to change several bits of code throught the site and CSS. Ideally if you want to change the logo only, keep it the same size, since the template has been designed for that size, and it usually looks best at that size...