It would appear to me from my testing that any rasters other than geotiffs that are in Geographic projection are NOT supported. When I generate a WGS 84 .proj file from ESRI ArcMap it contains:
GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433],AUTHORITY["EPSG",4326]]
ESRI runtime will only process rasters that also include the specification of the projection. For example,
PROJCS["UTM_Zone_11_Northern_Hemisphere",GEOGCS["GCS_WGS_1984",DATUM["D_WGS84",SPHEROID["WGS84",6378137,298.257223560493]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-117],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["Meter",1]]
Same goes for ArcMap. If anyone has an example of a jpeg, png, or jpeg that is spatially referenced in Geographic projection and loads successfully in ESRI runtime or ArcMap - I would greatly appreciate seeing the expected .prj file format.