As mentioned above:
Edit the file /themes/theme1224/css/global.css and just add the code supplied above.
Add:
#header_logo img { margin-top: 50px; margin-left: -19px; }
You can put this basically anywhere in the file as long as it is right after a }
You can change the 50px to suit, that will move the image down
You can change the -19px to suit. that moves it left. to move it right, just use a positive number like 10px.
The mention above to "centre" the logo is useful info for others reading this thread and may not be directly aimed at the OP.
#header_logo { display: block; margin: 10px 0 0 0; text-align: center; }
This code moves the image down 10 pixels and centres it on the screen.
I hope that clears things up for you.