If you're happy with the rest of the login page then you could just override the CSS selector for the logo… this is defined in the "login.css" file and the selector you'd need to override is: ".login-logo" which sets the background image.
If you do this by editing the file directly then you should bear in mind that the CSS file is loaded from the application theme - so depending upon the theme you're using you'll need to make sure you update the right file. Alternatively you can define an extension to import a new CSS file and override that selector in your own file (which would then be applied across all themes).
I'd probably recommend just starting off by placing your application logo image somewhere in the deployed Share application folder and using a Web Inspector/Firebug to dynamically update the CSS and see if you can get some results that you're happy with.