It is clearly a bug in the driver for package
dvips.def: ok for PostScript images, but clipping is not supported for bitmap images.
xetex.def: Clipping is not supported at all.
dvipdfm.def: The image is not trimmed, but distorted in the final area.
dvipdfmx.def: The whole image is put in the final area without distortion, but
empty space is put above the small image.
A remark to
keepaspectratio: It has a meaning only if both the
height are specified. Thus the setting and values of
keepaspectratio does not matter here.
There is a solution for
pdfTeX is used as TeX compiler (for DVI mode). Package
bmpsize fixes as side effect the
defective drivers. And the package improves the bitmap inclusion making separate bounding box files obsolete. The driver
xetex.def cannot be fixed this way, because XeTeX misses
primitives from pdfTeX (especially
\pdffiledump), needed by