Hi Michael, you can explore the internals of the DimensionReducerFunction to check the preprocessor—ideally you should be able to do

Information[_DimensionReducerFunction, "FeatureExtractor"]

but we have not hook it up there yet. In the meantime you can check what the internal processor is doing (hover over each processor to see more info)

reducer[[1, "Processor"]]

and apply it to an image

reducer[[1, "Processor"]] @* reducer[[1, "Preprocessor"]] @ RandomImage[]

Remember to use DimensionReduction instead of DimensionReduce in order to get the function and not the reduced data directly.