I don't understand 100% what images you want to generate. I assume you want an Entity-Relationship diagram in a particular image format, right?
There is no such built-in feature like that in LLBLGen Pro. What you can do is:
A. Write a plug-in that uses the model information in your project and with some 3rd. party tool, generate an image.
B. Write a piece of code that consumes the project file and -somehow- produces an image file. This piece of code could be a console application that you can use in command line. Just be aware that any written code that uses the LLBLGen core api assemblies must be run in a valid licensed LLBLGen installation machine. Read the EULA.rtf located in the installation folder for more information.
C. There are some E-R model tools that allows you to write plug-ins to import information and display it in a diagram, such as Visual Paradigm.
D. Since your LLBLGen model is in sync with your DB, you can use any diagram plug-in for your Database software that produces what you want to do. This would require more research about that of course.
(Edit)
I just realized that you can create "Model Views" in LLBLGen Pro Designer, and you can export the model view to an image file from the Designer. What I think you can't do is do this from command line.