You don't have to overwrite them, just rename them to something else (like the theme name, so it will be easy to find them later). Then upload your new image/s.

And not related to the subject, but allmost, Also notice that the your_account images are located in "themes/your_theme/images/menu" folder and change automatically for each theme. If you ever get a theme that uses the default your_account images (located in the "modules/your_account/images" folder) it is because the folder "menu" is missing from themes/your_theme/images/menu