IN ORDER TO CHANGE YOUR HEADER/ BANNER:
*You have to be using a custom layout, not a generic LJ layout. The community
refutedrefuted (and LJ won't let me link it, sorry) is an awesome place to find cool layouts (both of mine are from there), and they have easy instructions for changing your layout. And once you've added a banner, BAM, awesome LJ. :D :D
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Stuff here. )