first thing you need to do is go to the FTP location of those files, which varies from user to user, but after you get into the wordpress directory is located at
/wp-content/themes/YOURTHEMENAME
CHMOD all those files to 777
now, go into your wordpress control panel and to PRESENTATION>THEME EDITOR
now go to Sidebar
there you will find all your sidebars
now look for things like
those are your sidebar contents, and each one will be about the same and a good theme will have them seperated for you, like this:
YOU WANT TO PUT YOUR CONTENT HERE, before or after the sidebar item
make sure it is the right sized banner or item, because if not, it will fuck up the formatting of the blog