There are 3 things you need to be aware of when exporting builds:

1. By default, images are not included in the resource system
2. By default, the Godot export incorrectly installs the webview
3. By default, extension .html is ignored during export (change this in the export settings)

Visit the relevant help sections for the fixes.

1. Images
2. Fix paths: Windows/Linux
3. Fix paths: macOS