Can't paste new URL to change an image

Hi, this is my very first day using WordPress so this question is going to expose my complete lack of knowledge….

I’m trying to input an image onto my homepage using the ‘home page control’ feature. However, when I go to delete the old URL address so I can input the URL of my new chosen image I’m sent back to the previous page…. The current URL is highlighted in grey as opposed to the ‘title’ section underneath which is highlighted white (I can change and delete that for some reason?

Can anyone help please? ??