João Luís

WordPress allows you to define a site logo (through Jetpack) and a site icon, why would you need to upload another image to use in your login page?
This plugin allows you to forget the login page. Just activate it and all the magic is done for you. No extra steps required.
The plugins uses the site logo if it exists, otherwise it will use the site icon. If none of them exist, the plugin will not touch your login page.

JL Login Logo

Uses your site logo (defined through Jetpack) or your site icon as the logo on the login form.

Mark Jaquith

The <a href="https://wordpress.org/extend/plugins/jetpack/" rel="ugc">Jetpack</a> plugin by Automattic does something sneaky: it auto-activates new Jetpack modules. Instead of allowing you to opt in to new modules, Jetpack cheerfully activates them without your permission. These modules can change how your site works and looks.
Manual Control puts a stop to that unwanted behavior. It prevents auto-activation of new Jetpack modules.
Metal texture in the plugin header image by <a href="http://urbandirty.com/gallery/photo/2568483179-m" rel="nofollow ugc">Urban Dirty</a>.

Manual Control for Jetpack

Jetpack auto-activates new modules, without asking your permission. This plugin stops that.

JL Login Logo vs Manual Control for Jetpack