The UI would expect the new Background-Image or/and Logo-Image to be in <ui-host>/resources/img/ with the names:
bg.jpg for the new background (<ui-host>/resources/img/bg.jpg)
logo.png for the new logo (<ui-host>/resources/img/logo.png).
If there aren’t any images in this location, UI will consider the default background and logo images for background of archive UI and logo displayed at bottom right respectively. If you are using the dockerized version of the Archive and UI, you have the possibility to paste those images in a sub-folder ( ui/configuration/resources/img ) of the mounted folder of the archive UI (default defined as var/local/dcm4chee-arc) respectively in /var/local/dcm4chee-arc/ui/configuration/resources/img.
Make sure to change the logo size is 148x42px it will otherwise not load with the default css
For some reason with this approach my result is having both the default logo and the custom logo apper on top of each other.
To overcome this you can change your logo.png files background color to white so then it will overwrite the default logo.
There is some info on the css here
If you use the Dockerized version, you navigate to the mapped Docker location ( in Linux, it would be something like /var/local/dcm4chee-arc ). If the UI is served as a separate image, you should see an ui folder. In there you can find a resources folder, where you can create a css folder and put your CSS file named style.css in there.