I suggest using gdal.Translate to convert your PNG to a georeferenced GeoTIFF:
from osgeo import gdal
bounds = [west, north, east, south]
gdal.Translate('output.tif', 'input.png', outputSRS='EPSG:4326', outputBounds=bounds)
Use the coordinates from your bounding box as the outputBounds, making sure the SRS is correct (probably 4326). You can then include ...
To add a new features as a new layer in a map made by Mapnik, you have to :
1) Add a new layer in your project.mll file (assuming the data is within a shp)
"extent": "-20037508.34 -20037508.34 20037508.34 20037508.34",
"srs": "+proj=lcc +lat_1=51.16666723333333 +lat_2=...