Upload Image to S3 throwing error while decoding image via base64

I am trying to upload image to my s3 bucket , below is the lambda function:

import json
import base64

import boto3
client = boto3.client('s3')

def lambda_handler(event, context):
    # TODO implement 
    print('Started')
    imageDecoded = base64.b64decode(event['body'])

    response = client.put_object(
    Body = imageDecoded,
    Bucket = 'sulabhtest',
    key = "thisKey.jpg",
    )


    return {
        'statusCode' : 200,
        'body' : json.dumps('Image Uploaded')
    }
  

and in cloud watch i am seeing error at line : imageDecoded = base64.b64decode(event['body'])

that image contains ASCII character. how to resolve this error? i am uploading png/jpg image from postman

[ERROR] ValueError: string argument should contain only ASCII characters
Traceback (most recent call last):
  File "/var/task/lambda_function.py", line 10, in lambda_handler
    imageDecoded = base64.b64decode(event['body'])
  File "/var/lang/lib/python3.9/base64.py", line 80, in b64decode
    s = _bytes_from_decode_data(s)
  File "/var/lang/lib/python3.9/base64.py", line 39, in _bytes_from_decode_data
    raise ValueError('string argument should contain only ASCII characters')