Black and White thumbnails

You can use the php GD library which you most likely already have on your server since wordpresses uses it.

You can filter the image using imagefilter specifically IMG_FILTER_GRAYSCALE and/or IMG_FILTER_CONTRAST.

For example 

imagefilter($im, IMG_FILTER_GRAYSCALE);
imagefilter($im, IMG_FILTER_CONTRAST, -100);

The hover would have to been done in javascript.

A much simpler solution would be to use the html5 canvas tag or a javascript image library like pixastic and wrestle with making it browser friendly ( not that hard using one of the html5 js kits)

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