The theming app enlarges my background image

To shorten the loading time, I usually reduce the background photos to 96 DPI. But when I load the photo to the server via the theming app, the photos are enlarged to 240 DPI again. The photo has not the size of 300 KB as I wanted, it is over 2 MB. The same happens when I do this via user theming.

What is the reason for this?

Nextcloud version: 25.0.1
Operating system and version: Ubuntu 20.04.5
Apache version: 2.4
PHP version: 8.0.25