This one gets asked a lot in the google sites help forum
Ok..... there are possibly a few ways the logo could be attached to the top of the page
Click on more actions > manage site > colours and fonts > scroll down and click on Site header background image and see if that is where the logo is
If the logo is not there ..... click on more actions > manage site > site layout > change logo and see if it is there ......
And last but not least, the template logo could be part of the background image.
If this is the case you will need to click on view, save the image to your pc and then edit the image by placing your logo over the top for example in a program such as photoshop.
Let me know if this sorts out the situation for you