Just a couple of points Miguel -
1. This is the English language forum, so its helpful to ensure the request is in that language so it can be understood by all
Google translate:
I would like to delete the white box to be able to see the logo of the login page, attached image.
2. If you use a tool such as chrome developer tools or firebug it will give you the css selector for that area (I believe its .loginbox, but I'm not at my own PC right now to check) and then you can make the background colour transparent by adding a css style to the customcss setting within Essential - something like
.loginbox {background-color: transparent;}
Note - this is from memory and NOT tested! so please check it out before applying to a live site.