As noted here: http://gis.stackexchange.com/questions/18280/batch-convert-geotiffs-to-kmz-files, an undocumented method for conversion to KML exists in GDAL’s gdal_translate as format KMLSUPEROVERLAY. To do this with an existing image, such as a geotiff, your command will be the following format (on Windows):
C:OSGeo4W64>gdal_translate -of KMLSUPEROVERLAY FORMAT=JPEG
with an example path, this looks like:
C:OSGeo4W64>gdal_translate -of KMLSUPEROVERLAY D:downloadstile177_12orthos177.tif D:downloadstile177_12orthos177.kml -co FORMAT=JPEG
This will output a tiled images with a KML to tie them all together.