Flask: How to get url for dynamically generated image file?

from flask import Flask, redirect, url_for

app = Flask(__name__)
@app.route('/')
def index():
        generate_img("test.jpg");
        return '<img src=' + url_for(filename='test.jpg') + '>'

generate_img() will output a random image to the current directory (same folder as this script).

I am getting 404, I navigate to mydomain.com/test.jpg but it's not there.