  We use cosmological hydrodynamical simulations of Milky-Way-mass galaxies
from the FIRE project to evaluate various strategies for estimating the mass of
a galaxy's stellar halo from deep, integrated-light images. We find good
agreement with integrated-light observations if we mimic observational methods
to measure the mass of the stellar halo by selecting regions of an image via
projected radius relative to the disk scale length or by their surface density
in stellar mass . However, these observational methods systematically
underestimate the accreted stellar component, defined in our (and most)
simulations as the mass of stars formed outside of the host galaxy, by up to a
factor of ten, since the accreted component is centrally concentrated and
therefore substantially obscured by the galactic disk. Furthermore, these
observational methods introduce spurious dependencies of the estimated accreted
stellar component on the stellar mass and size of galaxies that can obscure the
trends in accreted stellar mass predicted by cosmological simulations, since we
find that in our simulations the size and shape of the central galaxy is not
strongly correlated with the assembly history of the accreted stellar halo.
This effect persists whether galaxies are viewed edge-on or face-on. We show
that metallicity or color information may provide a way to more cleanly
delineate in observations the regions dominated by accreted stars. Absent
additional data, we caution that estimates of the mass of the accreted stellar
component from single-band images alone should be taken as lower limits.
