Hi Katrice,
You can change the image in logon page by customizing the settings in SICF. if you require it for all WD applications, choose service name WEBDYNPRO, if you want for specific application, then choose your own WD application.
Please refer the below link
The above link explains you about changing the copy right text, but you can also change the logo, image, other details
Hope this helps you.
Regards,
Rama