Add option to download just the smaller images for training, but still record checksums of full sized originals?

[egrace479]

It [downsample] was intended as the smaller versions used for model training, so you get the original images for reference, processing, etc., but can also just immediately get training images.

It might be useful to give the option to keep only resized images (while still getting a checksum of the full-sized original). This could go into an issue for a future version though.

Originally posted by @thompsonmj in #1 (comment)