Small modification of my Wordpress site

Hi, When using wordpress "add media", then selecting and uploading files (in my case it's always pictures) and selecting "Link thumbnails to: attachment page" in the "gallery settings" , then saving, inserting the gallery and publishing the post. When you browse the gallery in your browser and click on an image, you're taken to the attachment page with this picture. I'd like to be able to insert an advertising banner and text link on the top and bottom of that picture. I'd also like you to tell what you changed so i'll know this in the future. I'm using Artisteer generated theme. I'm attaching jpg to give you a better picture. I'd also like the person to insert the code clean so it won't slowdown or break my site in the longer run. Let me know if you're interested.