I'm not very good at working Wordpress (or anything else) and I have several questions. Could someone tell me why this might have happened?

I would like to ask what would cause a photo from my media library to attach itself to a post that I didn't insert. I tried changing themes, but it just tagged itself along. It ended up that I had to delete the picture from my media library completely.

When I found another theme that I liked, I wanted to replace the default pictures in the masthead with some of my own. When I did this, the pictures also ended up trailing the end of the page ahead of my first posts.

Needless to say, I had a very frustrating afternoon. I'd like to avoid having this happen to me in the future.