Ok, find this line of code:
Code:
[img]{$ImagesDir}/companyname_small.gif[/img]
simply replace companyname_small.gif with the name of your image (i.e. logo.jpg or whatever the name of your image is. Then, upload that image to the skin1/images directory and you are done
FYI, that default image is 159 x 51. And don't worry, you can't screw it up, there is always a backup in the skin1_original folder. Although I do suggest saving a copy of the template before you edit it anyway, always pays to be safe. Good luck!