Unable to import module 'lambda_function': No module named 'lambda_function

Following a tutorial I'm attempting to create a lambda function that listens for an image being uploaded to an S3 bucket and then generates a thumbnail and places it in another S3 bucket.

I've done the following actions:
-The lambda function is written in python and uses a python imaging library. The python file and library are zipped and uploaded as the function code.
-Defined an environment variable: key: DEST_BUCKET value: ds-animalpics-thumb
-Added a trigger event (called objectupload) to the originating S3 bucket which notifies the lambda function when a PUT operation occurs.

I get the following error when I attempt to test the lambda function. What I am I missing?

{
"errorMessage": "Unable to import module 'lambda_function': No module named 'lambda_function'",
"errorType": "Runtime.ImportModuleError"
}

The python file and execution role are below. Please let me know if there is anything else helpful to upload.

Python file called lambda_handler.py:

import os
import tempfile

import boto3
from PIL import Image

s3 = boto3.client('s3')
DEST_BUCKET = os.environ['DEST_BUCKET']
SIZE = 128, 128

def lambda_handler(event, context):

    for record in event['Records']:
        source_bucket = record['s3']['bucket']['name']
        key = record['s3']['object']['key']
        thumb = 'thumb-' + key
        with tempfile.TemporaryDirectory() as tmpdir:
            download_path = os.path.join(tmpdir, key)
            upload_path = os.path.join(tmpdir, thumb)
            s3.download_file(source_bucket, key, download_path)
            generate_thumbnail(download_path, upload_path)
            s3.upload_file(upload_path, DEST_BUCKET, thumb)

        print('Thumbnail image saved at {}/{}'.format(DEST_BUCKET, thumb))

def generate_thumbnail(source_path, dest_path):
    print('Generating thumbnail from:', source_path)
    with Image.open(source_path) as image:
        image.thumbnail(SIZE)
        image.save(dest_path)

Execution role:

{
  "Version": "2012-10-17",
  "Statement": [{
      "Effect": "Allow",
      "Action": [
        "logs:CreateLogGroup",
        "logs:CreateLogStream",
        "logs:PutLogEvents"
      ],
      "Resource": "arn:aws:logs:*:*:*"
    },
    {
      "Effect": "Allow",
      "Action": [
        "s3:GetObject"
      ],
      "Resource": "arn:aws:s3:::ds-animalpics-upload/*"
    },
    {
      "Effect": "Allow",
      "Action": [
        "s3:PutObject"
      ],
      "Resource": "arn:aws:s3:::ds-animalpics-thumbs/*"
    }
  ]
}