I usually scan at a higher dpi and not worry about the original image size so I have good starting images to work with. Then I run it through my regular graphics program once I determine what physical size it needs to be for the ebook and experiment with what format gives the smallest file size and still maintains good image quality. Usually gifs and pngs will give better quality than jpgs and lower file size.
I also use an old program that is probably now free, ULead SmartSaver Pro, that reduces the file size even further without quality loss.
Another trick I often do when I need to reduce the physical size of the image (what comes direct from the scan is always much too large for use within an ebook): Instead of using your graphic program's reduce size tool, try zooming the image out until it's the correct size you want, take a screenshot and crop to the image only. That quite often gives a much clearer image than the reduction tools even in Photoshop.
After that I still run it through SmartSaver Pro though to reduce the file size.
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