Fair enough.
As well as the solution that has already been provided (replace the file with your own), you could also edit the Admin Template.
Admin Templates are only visible with Debug mode enabled.
Admin Templates don't have any sort of versioning / style system such as public templates. This basically means that any changes you make to Admin Templates would be overwritten during XF core upgrades. So the easiest thing to do here would be to create an Admin Template Modification. Admin Template Mods should remain active across upgrades as long as the content of the template doesn't change considerably.
The admin template is LOGIN_PAGE.
You should be able to find ..\XenForo.png and replace that file path with the path to your own logo.