Something funny is happening here!
Before testing something (I'll say later) I would say the things below:
>>
Maybe I haven't expressed myself properly.
I don't want to change the login background picture, I want to change the boot picture (the gray apple that appears at the startup, when you turn on your mac) and its size.
That gray apple file (BootLogo.png) is located at the same place as the DefaultDesktop.jpg file is. See image below.
The point is that I don't know how to change the boot picture size (to fullscreen, for example).

<<
But just after writing all that, I've thought that this file (BootLogo.png) could habe been created by BootXChanger. So then, I opened that program and set the boot image to the default (the gray apple), deleted that .png file and restarted the computer.
I noticed that the original gray apple appeared at the startup (boot) screen without troubles and that the BootLogo.png file hadn't been restored (as if the OS did not need that file).
After that, I opened the BootXChanger.app to change the boot image again and the BootLogo.png file reappeared.
So, now, I have 2 questions:
- the original one; how do I set a fullscreen boot image?
- and; if the BootLogo.png file doesn't exist, how do I change the boot image without having BootXChanger?