(simply put) people perceive more vertical resolution (you can "see" more horizontal lines) than horizontal ("see" more vertical lines).
instead the common accepted dimetions of 300x170 with X:1.6,Y:1.6
my settings are:
254x200
(pixel width is X:1.88 and Y:1.36)
don't forget to use USM (unsharp mask) after resizing. I usually use r:0.6 @ 600% gain
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http://www.alienminds.ca/PSP.bmp