Download and install the plugin called Custom Admin Branding.
Browse the following directory to download Admin panel PSD template to guide you while designing :
Then upload your images in this folder or simply replace the current ones. Remember to make your header and footer images in the .png format while your other login images should be in .jpg format:
Once uploaded you are done.
Another plugin that does the same job is called BM Custom Login
For those users who are looking to avoid using the plugin then try this method shared by Michael from Problogdesign which is also a great substitute.