Going down to 400x400 pixels is a terrible idea if there are fine lines and other details such as text, 400x400 represents a tiny amount of visual information and is useless for any other purpose than a low-res preview: but if that's what you're after, read on.

If the images are _literally _black and white (rather than grayscale), convert them to bitmap mode (i.e. where each pixel is either black or white) _before _reducing the size. This should avoid any anti-aliasing artefacts (which is probably what's causing the blurry text), but this process (pixel decimation) is going to be hit-or-miss as far as retaining the detail you want.

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Windows 10 x64 Pro - Intel Xeon E5450 @ 3.00GHz (x2)

PoserPro 11 - Units: Metres

Adobe CC 2017