aws lambda isBase64Encoded is not working for me

so I have this set in my api gateway settings:

the following code in my lambda:

        return {
            "statusCode": 200,
            "body": image_in_base64,
            "headers": {"Content-Type": "image/jpeg"},
            "isBase64Encoded" : True
        } 

and this set in the gateway config:

and when I call from a browser - I'm getting a jpeg image type - but the content length shows that the content is still base64 encoded - the gateway is never decoding it regardless of what I set isBase64Encoded to.

What am I doing wrong?
[And this HAS to work from any browser - so I can't control the "Accept" parameters]