FLASK: HTML cannot retrieve my images from my image resources folder? Follow

have the html code snippets:

<img src="/images/sirdis-full-size.png" >

and

<link rel="stylesheet" type="text/css" href="stylesheets/sidenav.css">

but I get the error 

127.0.0.1 - - [09/Feb/2020 17:47:48] "GET /stylesheets/sidenav.css HTTP/1.1" 404 -
127.0.0.1 - - [09/Feb/2020 17:47:48] "GET /images/sirdis-full-size.png HTTP/1.1" 404 -

What am I doing wrong and how to fix it?