  [ABRIDGED] We are undertaking an imaging study of local star-forming galaxies
in the Lyman-alpha (Lya) emission line using the Solar Blind Channel (SBC) of
the ACS onboard the HST. Observations have been obtained in Lya and H-alpha
(Ha) and 6 continuum filters between ~1500AA and the I-band. Previously (Hayes
et al 2005) we demonstrated that the production of Lya line-only images in the
SBC-only data-set is non-trivial and that supporting data is a requirement. We
here develop various methods of continuum subtraction and assess their relative
performance using a variety of spectral energy distributions (SED) as input. We
conclude that simple assumptions about the behavior of the ultraviolet
continuum consistently lead to results that are wildly erroneous, and determine
that an SED fitting approach is essential. Moreover, a single component stellar
or stellar+nebular spectrum is not always sufficient for realistic template
SEDs and, in order to successfully recover the input observables, care must be
taken to control contributions of nebular gas and underlying stellar
populations. Independent measurements of the metallicity must first be
obtained, but details of the initial mass function play only a small role. We
identify the need to bin together pixels in our data to obtain signal-to-noise
(S/N) of ~10 in each band before processing. At S/N=10 we can recover Lya
fluxes accurate to ~30% for Lya lines with intrinsic equivalent width [W(Lya)]
of 10AA. This accuracy improves to ~10% for W(Lya)=100AA. We describe the image
processing applied to the observations presented in \"Ostlin et al. (2009) and
the associated data-release. We also present simulations for an observing
strategy for an alternative low-redshift SBC Lya imaging campaign using
adjacent combinations of long-pass filters at slightly higher redshift.
