When specifying lengths in "pixels" in CSS, note that they may or may not correspond to actual screen pixels. In particular, the
CSS2.1 spec says:
px: pixel units — 1px is equal to 0.75pt.
and since 1pt=1/72in, that assumes that all devices are 96ppi.
Of course, an image's (bitmap, not vector) dimensions are most meaningful in real pixels, it is only when setting the image size in CSS that this fixed-size pixel enters play.