In addition to using test images, we also use photos to check the effect of filters in the real world. We generally use photos from Unsplash as our test samples, but you can also build your own test set. In general, a good test set should cover a wide range of colors and genres. However, depending on the filter and intention of the creator, the colors and genres of the photos in the set could also have a consistent manner.
Here are links to some images we’ve found quite helpful in our testing process.