Why are you making it a custom field? Put the code in the sidebar of the template (looks like that's what you have), and style the image to "margin:0px auto;" to center it...
I don't know that plug in or whatever part of wordpress you're using, but from the layout I would say, go to Appearance>Editor and select the "sidebar.php" file- you should be able to see where it goes there and it won't repeat...
Just
from my cursory glance...
:D