I've not tried this, but this is the approach I'd take:
1) Forget all about python to start with
2) Find out how to print from the command line (a quick google suggests installing cups and using the lpr command) and work out what command you need to run, including options to scale image etc.
3) Time for python...
4) Download file
5) Use subprocess module to call the "lpr" command (plenty of examples of using subprocess on this forum)
RPi Information Screen: plugin based system for displaying weather, travel information, football scores etc.