Answer
The APIMAGE or separate raster option for ArcPress writes all raster information in the map to a separate file. ArcPress then only rasterizes the vector information. The separated raster information is added afterwards.
It was originally designed for use with ArcInfo workstation *.gra files. It does not work with *.eps file input.
APIMAGE has the added benefit of applying color adjust and saturation options to the vector and/or raster information in the map.
Note:
APIMAGE or separate raster is no longer available in ArcPress for ArcMap ArcGIS 9.x and above. It is only available in ArcPress for ArcInfo Workstation 9.x and above.