Ah would not leave that nukemods.com logo there 8) only kidding
If you right click on the logo & properties, it should give you something like this
yoursite/themes/yourtheme/images/logo
or
yoursite/themes/yourtheme/forums/images/logo
upload your image to either directory and rename to logo
Theres just one problem that might occur, your image might be a .gif .jpg or other, in that case you need to open your themes/header or theme.php file and search that for your logo.something and rename that to match yours etc..