How to control which pages show a header pic

I’m a newbie to WP, and have chosen a template that has a large picture as the header that shows up on each page. On one of my pages, I’ve created a picture gallery which shows several pictures as thumbnails in the body of the page, below the header. When you click the thumbnail of a picture to open it in full view, it opens but the header is so big, you have to scroll way down the page to see the full picture. I’m trying to make it so when you open a pic, thats what you see. In other words, I want to learn how to make it so that header picture only shows up on pages that I want it to. I’m hoping its a simple fix. Any advice would be appreciated.