Error message in Jupyter Notebook with json

[snippsat]

The problem is that you have bytes b'something' in dictionary data.
JSON format only supports Unicode strings.
As mention bye @volcano63 you overwrite format,but also bytes is a used name bye Python.

>>> format
<built-in function format>
>>> bytes
<class 'bytes'>

>>> # A name that not used give NameError
>>> foo
Traceback (most recent call last):
  File "<string>", line 428, in runcode
  File "<interactive input>", line 1, in <module>
NameError: name 'foo' is not defined 

You should also show what you call function with,now do i see that in error message that it's png image.
Since Base64 encodes bytes to ASCII-only bytes,you can use that codec to decode the data so it can be dumped to JSON.

import base64
import json
import subprocess
 
def output_image(name, file_format, img_bytes):
    image_start = "BEGIN_IMAGE_f9825uweof8jw9fj4r8"
    image_end = "END_IMAGE_0238jfw08fjsiufhw8frs"
    data = {}
    data['name'] = name
    data['format'] = file_format
    encoded = data['bytes'] = base64.encodestring(img_bytes)
    # Fix
    data['bytes'] = encoded.decode('ascii')     
    print(image_start + json.dumps(data) + image_end)
    
output_image('logo.png', 'png', open("logo.png", "rb").read())