You use something like
from flask import send_file
@app.route('/get_image')
def get_image():
if request.args.get('type') == '1':
filename = 'ok.gif'
else:
filename = 'error.gif'
return send_file(filename, mimetype='image/gif')
to send back ok.gif or error.gif, depending on the type query parameter. See the documentation for the send_file function and the request object for more information.