Thanks! And I think I found the problem: Before you’re returning pic_json, you’re calling json.dumps on it, which turns it into a string. However, examples added to the database are expected to be just a regular list of dictionaries. So you can drop that line 
I also just noticed that add_examples currently expects the examples to already have hashes set (an "_input_hash" representing the input data, e.g. the image, and a "_task_hash" representing the specific question, e.g. the image with a given label). You can either generate them yourself, or let Prodigy do it for you:
from prodigy import set_hashes
# in your code
pic_json = set_hashes(pic_json)
return pic_json