RHESSI images are output in units of photons/cm^2/sec/asec^2, Since the diagonal SRM was used to convert from count flux to photon flux, there are no corrections for effects such as k-escape and compton scattering (the off-diagonal elements of the detector response matrix). Spectra from image cubes should not be analyzed at low energies in the default mode (i.e. with spex_image_full_srm set to 0.
If the spex_image_full_srm option is set, then OSPEX calculates the diagonal SRM and applies to the spectrum to convert back to counts/cm^2/sec/asec^2 (basically reversing the conversion from counts to photons applied in the RHESSI image object). A full SRM incorporating all off-diagonal elements is calculated for each attenuator state that occurred during the time of the image cube, and stored in the spex_drm object for use in converting the spectrum to photon rate or flux. You will notice that if this option is set (spex_image_full_srm=1), you have the option to plot in count or photon space, whereas you can only plot in photon space if the option is not set.