  We used 8.4 GHz VLBI images obtained at up to 35 epochs between 1997 and 2005
to examine the radio structures of the main reference source, 3C 454.3, and two
secondary reference sources, B2250+194 and B2252+172, for the guide star for
the NASA/Stanford relativity mission Gravity Probe B (GP-B). For one epoch in
2004 May, we also obtained images at 5.0 and 15.4 GHz. The 35 8.4 GHz images
for quasar 3C 454.3 confirm a complex, evolving, core-jet structure. We
identified at each epoch a component, C1, near the easternmost edge of the core
region. Simulations of the core region showed that C1 is located, on average,
0.18 +- 0.06 mas west of the unresolved "core" identified in 43 GHz images. We
also identified in 3C 454.3 at 8.4 GHz several additional components which
moved away from C1 with proper motions ranging in magnitude between 0.9c and
5c. The detailed motions of the components exhibit two distinct bends in the
jet axis located ~3 and ~5.5 mas west of C1. The spectra between 5.0 and 15.4
GHz for the "moving" components are steeper than that for C1. The 8.4 GHz
images of B2250+194 and B2252+172, in contrast to those of 3C 454.3, reveal
compact structures. The spectrum between 5.0 and 15.4 GHz for B2250+194 is
inverted while that for B2252+172 is flat.
  Based on its position near the easternmost edge of the 8.4 GHz radio
structure, close spatial association with the 43 GHz core, and relatively flat
spectrum, we believe 3C 454.3 component C1 to be the best choice for the
ultimate reference point for the GP-B guide star. The compact structures and
inverted to flat spectra of B2250+194 and B2252+172 make these objects valuable
secondary reference sources
