Adding banner image to custom theme built by different developer

I’ve been given the job of taking over a site developed by a different developer. The task I need to accomplish seems simple, add a banner image I’ve created to the top of the blog page. The blog page is using a template named “ideas.php.” I’ve opened that page to reveal there is no code on the actual content page, but there is code on the template “ideas.php.” So, on that ideas.php template, after the call to the header, I’ve added my image, and yet it will not display in the browser at the top of the blog page. I’ve tested my code, and the new banner image appears when I created a new page, and apply the same template “ideas.php.” The only difference in the original blog page and this new page I’ve created for testing, is that the original blog page only displays 25 posts, and the new test page I’ve created is displaying over 100 posts. I’ve compared the two pages from within WP admin, and they seem to have identical settings, use the same template, yet one shows the correct number of posts and no banner, the other shows the banner, but too many posts. I need the same set of posts currently displaying on the blog page to also display the banner.

Any help is greatly appreciated.

The original blog page is here:

and the test page I created is here:

If I had a private way, I could send you the code for the template in question. Please advise on that.

Thank you,