If you want to keep your own logo on wordpress admin page, it is quite simple. It is very good for branding and total uniformity, if you are providing any solution to your clients!!. To change the wordpress logo on admin page here are the steps:

Open the folder “wp-admin”
Inside that, open the folder “images”
Find a file named “logo-login.png”. The size of the image is W:274 x H:63
Now, either you can open it in the photoshop and change it or you can re-size your image and give your image name “logo-login” and save in PNG format.
Upload on your server and you are good to go!!
Its easy. Isn’t it.