[meryn]

I'm not sure what resolution these images are, but if it's very high-res, it may make sense to make intermediate-sized versions first. I expect that would make recreating thumbnails (i.e. in a different size - or multiple sizes) much quicker.

Also, converting the (hi-res) images to JPEG before further processing may make the data somewhat easier to deal with. 25-30MB per image * 2 million would be 50-60 terabytes of data? Plus, how often would someone really benefit from having access to a 25-30MB TIFF vs a much smaller JPEG version in the same resolution? That alone could be reason to start with JPEG compression first.