How to get data from collection to workflow incovation


I have a workflow that generates reports e.g. as PNGs of my data. I want to use them in my final report that I generate using the workflow invocation. I understand that I can display PNGs using history_dataset_as_image(...) but how can I reach the PNGs within the collection if I want to display them all?
output_label[0] etc. don’t work so far. Any ideas?