Six BL Lac objects from the complete 1 Jy radio-selected sample of 34 objects were observed in cycle 5 with the Hubble Space Telescope (HST) Wide-Field Planetary Camera 2 (WFPC2) to an equivalent limiting flux of mu_I~26 mag arcsec^-2. Here we report results for the second half of this sample, as well as new results for the first three objects, discussed previously by Falomo and coworkers. In addition, we have analyzed in the same way HST images of three X-ray-selected BL Lac objects observed by Jannuzi and coworkers. The ensemble of nine BL Lac objects spans the redshift range from z=0.19 to ~1. Host galaxies are clearly detected in seven cases, while the other two, at z~0.258 (redshift highly uncertain) and z=0.997, are not resolved. The HST images make up a homogeneous data set with unprecedented morphological information between a few tenths of an arcsecond and several arcseconds from the nucleus, allowing us to rule out definitively a pure disk light profile in six of the seven detected host galaxies. The host galaxies are luminous ellipticals with an average absolute magnitude of M_I~-24.6 mag (with dispersion 0.7 mag), more than a magnitude brighter than L^* and comparable to the brightest cluster galaxies. The morphologies are generally smooth and have small ellipticities (ɛ<~0.2). Given such roundness, there is no obvious alignment with the more linear radio structures. In the six cases for which we have HST WFPC2 images in two filters, the derived color profiles show no strong spatial gradients and are as expected for K-corrected passively evolving elliptical galaxies. The host galaxies of the radio-selected and X-ray-selected BL Lac objects for this very limited sample are comparable in both morphology and luminosity.