AWS API Gateway POST request not returning output when sending image as payload

I have spent an lot of time, but still with no luck.

My objective is to send an image to the AWS Lambda function and do some image processing there and return the modified image back.I am using API Gateway. So, the problem operation for me are:

1. Sending the image data to Lambda server.
2. Returning the image from Lambda.

I am using POST API. And since now we can work with binary data in API Gateway, I followed the API Gateway setting part from this official blog.

My Lambda function is like this :

def lambda_handler(event, context):
    # TODO implement
    image = event['base64Image']
    return {
        "isBase64Encoded": True,
        "statusCode": 200,
        "headers":{
                "content/type": "image/png",
        },
        "body": "any string"
    }

And my call to the API python code is this :

headers = {
    'Accept': 'image/png',
    'Content-Type': 'image/png',
}

data = open('random.jpg', 'rb').read()
response = requests.post(' https://xxxxxxx.execute-api.us-east-1.amazonaws.com/prod', headers=headers, data=data)
print(response.content)

According to the blog, in the Integration Request I added the Mapping template image/png and also the template

{
    "base64Image" : "$input.body"
}

And in the API > Settings in Binary Media Type I added image/png.

What is happening now is, I am able to receive the image base64 data properly in my event[base64Image] object.

But the response result that I am getting is not coming. As you can see I am just returning a simple string but I get '{"message": "Internal server error"}' error. In place of "any string" in the result body I tried sending base64 data also directly but get the same error.

So, the problem boils down to how to receive result in POST Request when we send image data as payload.

I also followed this answer and accordingly in my Integration Response I selected Convert to Binary (if needed) option. But that also didn't help.

If I am not wrong then the problem is something related to application/json and image/png but I tried all permutation combination everywhere but nothing seems to work.

I am attaching the screenshots of the API Gateway setup :

INTEGRATION REQUEST

INTEGRATION RESPONSE

SETTINGS

The Lambda function in its own console is giving the output properly, so my problem right now boils down to receive any result from the lambda function using the POST API in this setup.

Edit : When I use a GET request to just send an image-> base64 output I am able to do without the changes mentioned in the blog. The Lambda function then is :

def lambda_handler(event, context):
    # TODO implement
    return {
        "isBase64Encoded": True,
        "statusCode": 200,
        "headers":{
                "content/type": "image/png",
        },
        "body": base64.b64encode(open('random.jpg','rb').read()).decode('utf-8')
    }

Here random.jpg is an image which is there in the Lambda zip folder only.

Correction : Its content-type and not content/type. (Error still there).