1. Login to your site via FTP or your preferred method and locate the file 'login.css'
which is found in your wp-admin/css/ folder. Open this css file in your
favourite editor.
2. In the login.css file look for the line:
background: url(../images/logo-login.gif) no-repeat;
and change it to your own image for example...
background: url(../images/YOURLOGO.gif) no-repeat;
NB: You will probably want to make the logo around 290 pixels wide.
3. For the more advanced users this is where you can customise the look of
the login anyway you like using CSS which I know you will. I just changed the
fonts to Helvetica to match my site.
To change the colour of the background (ie. the baby blue colour)
1. Open the colors-fresh.css file which is also found in your wp-admin/css/
folder.
2. Locate the TWO #eaf3fa colour's in this file and replace them with any
colour you like. I would recommend a light washed out or neutral colour. I
chose my site colour which is grey. (#EAEAEA)
3. Don't stop here, you can customise your log in screen as much as you
like! Mine was just a quick example.
Just a note to remember to backup these two CSS files (colors-fresh and
login.css) before you do any Wordpress updates as these updates will overwrite
these files and you will have to do it all over again!