arcpy select visible raster
import arcpy
mxd = arcpy.mapping.MapDocument("CURRENT")
df = arcpy.mapping.ListDataFrames(mxd, "*")[0] # grab 1st dataframe in map
#df = arcpy.mapping.ListDataFrames(mxd, "Main")[0] # grab dataframe named 'Main'
arcpy.MakeMosaicLayer_management(
'Orthophotos', # input mosaic
'test_mosaic', # output mosaic
'#', # Optional WHERE clause
df.extent # clipping box coordinates
)