If you already have your own images, you can use the Replicate model directly: https://replicate.com/jagilley/controlnet-scribble -- you can upload your image using the Replicate web UI or do it programmatically using the model's HTTP API.
I don't quite understand why some image generators are free and others charge credits, assuming the non-free ones aren't just goldrushing.
Is it just free until oops it gets too popular?
Of course running on rented/hosted GPU's, it's a simpler, but much more expensive story — basically however much you're paying for GPU instances to run Stable Diffusion divided by how many images you generate. :)