https://jpegxl.info/resources/jpeg-xl-test-page

This should get you started.

Hi again, Here’s some .jxl images encoded by me. These are lossless conversions from .jpg. This will only be viewable in a browser with .jxl support enabled.

Edit: Obviously I don’t own these images. If the photographer of one of these images sees this, don’t sue me. I downloaded these years ago and don’t remember where from.

The Waterloo Corpus is a collection of images thought good for testing of data compression methods: https://links.uwaterloo.ca/Repository.html. It’s pretty old now, though. There are many informal image collections.