  Generating images from very long baseline interferometric observations poses
a difficult, and generally not unique, inversion problem. This problem is
simplified by the introduction of constraints, some generic (e.g., positivity
of the intensity) and others motivated by physical considerations (e.g.,
smoothness, instrument resolution). It is further complicated by the need to
simultaneously address instrumental systematic uncertainties and sparse
coverage in the u-v plane. We report a new Bayesian image reconstruction
technique in the parameter estimation framework Themis that has been developed
for the Event Horizon Telescope. This has two key features: first, the full
Bayesian treatment of the image reconstruction makes it possible to generate a
full posterior for the images, permitting a rigorous and quantitative
investigation into the statistical significance of image features. Second, it
is possible to seamlessly incorporate directly modeled features simultaneously
with image reconstruction. We demonstrate this second capability by
incorporating a narrow, slashed ring in reconstructions of simulated M87 data
in an attempt to detect and characterize the photon ring. We show that it is
possible to obtain high-fidelity photon ring sizes, enabling mass measurements
with accuracies of 2%-5% that are essentially insensitive to astrophysical
uncertainties, and creating opportunities for precision tests of general
relativity.
