How do I use QGIS python console to iterate through simple layer queries and png outputs?

I'm really new to QGIS coding and I'm having a hard time finding (especially) the api classes/functions to carry out actions of the Query Builder window.

I'm basically trying to do this:

1. 
   

   Iterate from n=1 to 200...

   
   


2. 
   

   Query layerA where field_id = n

   
   


3. 
   

   Query layerB where field_id = n

   
   


4. 
   
   

   Query layerC where field_id = n

   
   


5. 
   

   Save as img_n.png

   
   

Pretty simple, I think, but it has become a real pain to do by hand in the GUI.