If you choose not to manage assets when you reference external HTML files, you have to get external pictures, media files, applets, and other assets to the Web server yourself.
You can upload the files to the server from wherever they reside using an FTP utility. Or, use the New File Asset command to copy assets into Assets view so they are published with the rest of the site.
In Assets view, from the Assets menu, choose New File Asset.
The File dialog appears.
Click Browse in the dialog to open an external asset you want to publish, and type a name for the asset. See Adding a File Asset.
Select Always publish file and click OK.
This causes the file to be published, even though it’s not listed as being in use.
In Publish view, position the asset in your directory structure so when it’s published, it can be found by any HTML page referencing it.
Because paths to unmanaged assets aren’t edited in the published HTML, you must position assets so the original paths work. See Customizing Your Site's Directory Structure.