Does your theme have any extra options? (Click themes and it should have OPTIONS: and then it should have links to options)
Otherwise you can try this.
If you go into your FTP, browse to your wp-content folder, and then go into themes. You should see a list of folders, pick the one which is named after your theme.
Now you are inside the folder, you will normally see something along the lines of sponsors.php. Open that in notepad and you should normally see space to put in sponsor links/images.
Make sure you back up the file before you do anything though.
