Thanks for the well documented package, it was quite straightforward to run src.run.digitize out-of-sample. That being said, the results are poor, and might indicate a generalisation issue (understandable if ECGs differ from PTB-XL).
To assist in understanding what the model expects, please can you provide simple image specifications (e.g. image size/resolution/colour format) and possible dimensions reflecting the ECG lead sheet and header. That would be a useful first step in ensuring any new input data somewhat conforms to model expectations.