Easy way to upload new login background image that won't be overwritten during an upgrade?

I want to change the background image for the login page. I found some instructions about changing the theme template using the backend editor but was not sure which file I would change and where I could change it. Please direct me to the right place if that is how I need to do it.

Basically I want to supply a different file instead of the current one at

https://www.drcarney.com/l/media/com_easysocial/images/bg-register.jpg

Thanks, Sean