This is the location of your current header image
/includes/templates/classic/images/logo.gif
If you only want to change the logo, then FTP the image of your choice to this location.
If you want to change the entire look and feel of the whole site then it might be worth investigating pre-built templates. There are some free ones available but for a small fee you can purchase a professionally built template for your store. Have a look at this site for some examples of what is available
http://www.dezinacart.com/