Thanks for the screenshot. In most cases it's a problem with path to the files. Also in that/your case.
Solution #1
Please use a FULL url to image file(s) with the domain name, then it will work on all subpages

OR
Solution #2:
use classic trick with two dots
url("../images/startseite/loginbtn.png")
both methods will work OK.
This is becuase for Joomla/WordPress/Drupal subpages have different path than used on homepage. And above methods allows CMS to see correct URL everytime. Subpages are like rooms in your flat/home, they are independed but still connected.