In Silk Central users may want to re-brand the login screen to use their own logo. This is mostly used in large organizations or academic sites so that the site is identifiable to users whenever they login.
For the image you want to use, you need to name this silkcentral.png, the reason for this is Silk Central is looking for this image on this page.
Once renamed in the installation folder you need to browse to:
C:\Program Files (x86)\Silk\Silk Central 17.0\instance_x_xxxx\wwwroot\silkroot\img\login
This folder will have the current silkcentral.png file. Replace the file in this directory and clear the cache on your browser, for the image to be retireved from the server again, and this should now appear on your login screen:
Each instance could have its own image for the specific department which is using that instance if required.