I've got a paid account, but I'm sure you can change it on plus? I'm fairly sure I had a banner before I paid for my LJ.

It depends what layout you have, some let you add in a background image or a banner to the page, some don't.

You can edit the colours, images etc if you go to the LJ homepage, then go Journal > Journal Style > Customise Selected Theme.

If your LJ layout doesn't have an option to add in a banner, you can use the Custom CSS to put one in. There's a good tutorial for it here.