AWS LambdaでS3からGetObjectを実行するとエラーが発生し対処法が分かりません.

エラー内容

{
  "errorMessage": "An error occurred (AccessDenied) when calling the GetObject operation: Access Denied",
  "errorType": "ClientError",
  "stackTrace": [
    [
      "/var/task/lambda_function.py",
      24,
      "lambda_handler",
      "raise e"
    ],
    [
      "/var/task/lambda_function.py",
      18,
      "lambda_handler",
      "response = s3.get_object(Bucket=bucket, Key=key)"
    ],
    [
      "/var/runtime/botocore/client.py",
      312,
      "_api_call",
      "return self._make_api_call(operation_name, kwargs)"
    ],
    [
      "/var/runtime/botocore/client.py",
      601,
      "_make_api_call",
      "raise error_class(parsed_response, operation_name)"
    ]
  ]
}

S3のポリシー

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowPublicRead",
            "Effect": "Allow",
            "Principal": "*",
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::fisourceimages/*"
        }
    ]
}

lambda関数

import json
import urllib.parse
import boto3

print('Loading function')

s3 = boto3.client('s3')


def lambda_handler(event, context):
    #print("Received event: " + json.dumps(event, indent=2))

    # Get the object from the event and show its content type
    bucket = event['Records'][0]['s3']['bucket']['name']
    key = urllib.parse.unquote_plus(event['Records'][0]['s3']['object']['key'], encoding='utf-8')
    try:
        response = s3.get_object(Bucket=bucket, Key=key)
        print("CONTENT TYPE: " + response['ContentType'])
        return response['ContentType']
    except Exception as e:
        print(e)
        print('Error getting object {} from bucket {}. Make sure they exist and your bucket is in the same region as this function.'.format(key, bucket))
        raise e

aws初心者のため、質問内容が適切かも判断つきません。
どなたかご教授お願い致します。