Header Issue on post pages in Twenty Eleven Theme

I wanted to hide the featured image on my posts so I added the following to my index.php file. However, now it is also hiding the header image at the top of some (but not all) of the pages (it should show the same header as you see on my homepage here: https://www.birdzofafeather.ca). Is there a way to get the header back on all post pages, but still hide the featured image?

I posed this question nine months ago and Jarret mentioned that if I can remove the code added that caused the issue I should be able to fix it using some CSS to hide what you want on certain pages. I would like to do that but don’t know how – and replies were closed, so I couldn’t respond. Any help on how to fix this would would be greatly appreciated.

The page I need help with: [log in to see the link]