To change the text in your sidebox headers, you need to make a copy of /includes/languages/english.php. For banner box 2, it's at about line 413-415 (in red):
Code:
define('BOX_HEADING_BANNER_BOX2','Have you seen ...');
Be sure to leave the single quote marks. Your altered file will go back into your override file at /includes/languages/custom. (This is the only file that will go into this folder.)
The contents of your banners are controlled in Admin/Tools/
Banner Manager. You can have an image (uploaded when you make the banner) or html. You can't have both on the same banner.