Static files

from flask import Flask, request, render_template, url_for
app = Flask(__name__)

@app.route("/")
def main():
    return render_template('main.html')

@app.route("/other")
def other():
    return render_template('other.html',
        img_path = url_for('static', filename='img/python.png'))

<h1>Main page</h1>
<img src="/static/img/python.png">
<p>
<a href="/other">other</a>

<h2>Other page</h2>
img_path: {{ img_path }}
<p>
<img src="{{ img_path }}">
<p>
<a href="/">main</a>

.
├── static.py
├── test_static.py
├── static
│   └── img
│       └── python.png
└── templates
    ├── main.html
    └── other.html

import static
import pytest

@pytest.fixture()
def web():
    return static.app.test_client()


def test_main_page(web):
    rv = web.get('/')
    assert rv.status == '200 OK'
    assert rv.headers['Content-Type'] == 'text/html; charset=utf-8'
    assert '<h1>Main page</h1>' in rv.data.decode('utf-8')
    assert '<img src="/static/img/python.png">'  in rv.data.decode('utf-8')
    assert '<a href="/other">other</a>'  in rv.data.decode('utf-8')

def test_other_page(web):
    rv = web.get('/other')
    assert rv.status == '200 OK'
    assert rv.headers['Content-Type'] == 'text/html; charset=utf-8'
    assert '<h2>Other page</h2>' in rv.data.decode('utf-8')
    assert '<img src="/static/img/python.png">' in rv.data.decode('utf-8')

def test_image(web):
    rv = web.get('/static/img/python.png')
    assert rv.status == '200 OK'
    assert rv.headers['Content-Type'] == 'image/png'
    with open("static/img/python.png", "rb") as fh:
        image = fh.read()
    assert image == rv.data