You can include an external resource (image, stylesheet or javascript library/file) in your screens by linking to it in your HTML content:
<script src='<appId>/my_script.js'></script>
<link rel='stylesheet' href='<appId>/my_stylesheet.css'>
<img src='<appId>/my_photo.jpg'>where <appId> is the unique identifier of the app.
The corresponding files should be located in the folder jtree/apps/<appId> folder. The src attributes must include the appId.
For example, see the pictures app.