This one was a fun one 
Workflow is as follows: In Revit create a Legend view with a single wall legend component. With Dynamo > Duplicate legend view per each wall type, change component to respective wall type, export views as images, import images and set parameter value, delete legend views
To avoid pasting all the Python scripts here is the .dyn file: Wall Type Image Creator.dyn (39.4 KB)
edit: ImageType.Load is from the package Prorubim. Any other custom nodes are Clockwork, Archi-lab and DanEDU
Definitely check your file size before and after running as @jacob.small mentioned it will increase file size. When I ran this with the images fit to 1000 pixels, for 26 wall types, it added about 500 KB