Need to replace 3 page titles with banner image, but effecting menu

I am using a purchased template from Mojo Themes. My client wants me to swap the page titles on 3 of his site’s pages with banner images. Problem is I know CSS, but not PHP.

First, I inserted the following code into the Page Title field, but then the top navigation menu picked up the image:

<img title=”Page Title” src=”Image URL” alt=”” width=”940″ height=”180″>

I saw that someone suggested using the following code in the header.php file, but when i did that, the alt text (not the image) appeared at the very top of the page, not in the page title area.

<?php if(is_page(‘Page Name or ID’)) : ?>
<img src=”[Image URL]” alt=”[alternative text]” />
<?php else : ?>
<!– normal title here –>
<?php endif; ?>

The pages I need to do this on are the Blog page, and then two static pages. The site header and top nav on all pages throughout the site should not be affected.

Does anyone know how to do this? I’m trying to figure out where the theme author’s code for the page titles resides and how to target it for those specific pages. The author is unreachable.