env.workspace = "My raster folder"
rasterList = arcpy.ListRasters("*.img")
print rasterList
#works
OutDir= "My output raster folder" + raster [0:7] + ".img"
#Here is my loop
for raster in rasterList:
    OutRaster = Con(IsNull(raster), 0, raster)
outCon.save(OutDir)
#this seems to be working but only for one of the 50 rasters in the List.