External markers not working with dynamically created PNGs

Hi,

I am trying to overcome the limited character count of markers by using dynamically created PNGs from shields IO. E.g this link will get you a little PNG label:

https://raster.shields.io/badge/ManyLetters-Value-121212.png

However, if I try to use that within a static map query like the following, no labels are visible:

https://open.mapquestapi.com/staticmap/v5/map?locations=New+York,NY|https://raster.shields.io/badge/ManyLetters-Value-121212.png&key=

Could you give some insight why this is failing silently?

Thanks,
Mike

Update: Also tried this with another dynamic service with the same result.