Catching Elephant is a theme for Tumblr.
Please look through all the answers here before asking a question. The same questions keep coming up. A lot of customisation can be done with custom css. Try googling css background for example.Catching Elephant is a theme by Andy Taylor
I just did a test and it’s working fine. Remember the feature will only be available if there is actually a high res version. And you need to click on the magnifying glass, not just the photo.
Or you may have broken it with all the customisation you’ve done.