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.