Header image won't go away.

Hi, I’m new to wordpress, and I know that topic sounds typical, but here is the deal.

I’m merrily building my website, https://www.groupenvelope.org , and want to change the featured image (rainbow parachute into our group logo). I do this. Doesn’t show up on website.

I try changing the header (replace the Blackbird theme header) and this works, but when I remove it (and delete any trace of the jpg. file from my media library, etc.)…but it just won’t go away.

This seems to have botched up my ability to try and install a different theme as well. When I do that, the header image STILL won’t go, and the newly installed theme is just a full of unformated text and blue question mark boxes.

I’m not an idiot. I have built websites in WP with other themes before and never encountered this. Anyhelp would be super appreciated. Thanks in advance. ^_^