downsampling a image to exactly half the width and height with Image.BILINEAR is broken. there does seem to be a off by one bug, the leftmost and topmost pixels aren't filtered at all, but are unmodified in the result.
i'm downsampling a image from 8192x8192 to 128x128 in 6 steps, always halfing width and height, using "Image.BILINEAR". (im = im.resize((size/2, size/2), Image.BILINEAR))
i've uploaded the source and result pictures here to illustrate the problem
(there doesn't seem a way to attach the files right here)