We report the first results of a Hubble Space Telescope (HST) program which performed WFPC2 multi-color imaging of a sample of A- and B-type supergiant stars in M31 and M33. This sample has previously been studied by us spectroscopically with the High Resolution Echelle Spectrograph (HIRES) on the 10m Keck-I Telescope. In particular, fits to the P-Cygni-like stellar Hα line profiles using state-of-the-art non-LTE unified model atmospheres have provided us with measures of the momentum in the stellar wind of each supergiant. When combined with metallicity information from our analyses of the HIRES absorption line spectra, these radiation-driven wind momenta allow us to determine the intrinsic stellar luminosities (and hence distances) via the Wind-momentum Luminosity Relation (WLR). The HST photometry reported here is a critical ingredient for this work, as it permits us to improve upon earlier ground-based apparent magnitudes for these objects (located in the crowded fields of M31 and M33 spiral arm OB Associations) and consequently to derive more precise distance moduli. Furthermore, the accurate photometric colors reported here will improve our determination of extinction to each object individually, via comparision with the intrinsic colors of the best-fit model atmospheres. Lastly, an implicit assumption of the WLR method is that the objects are highly-luminous single stars (not tight multiple star groupings unresolved from the ground), an assumption the HST imagery has allowed us to test. Support of this work from the Space Telescope Science Institute in grant GO-6651 is gratefully acknowledged.