The preview image for the HelpXplain project is always called preview.jpg and is always stored on the root level of the zip tree. To display the preview your application needs to open the .xplain file as a zip and extract preview.jpg.
The preview image always has exactly the same dimensions as the canvas defined in the HelpXplain project, even if the author has placed objects outside the canvas. For example, if the canvas is defined as 1920 x 1080 pixels, then that will also be the size of the preview image. This means you can use this image to define the relative dimensions/aspect ratio of the iFrame you use for embedding the Xplain in a web page. At least as a recommendation – the HelpXplain dimensions will always automatically adjust to whatever viewport it is displayed in.
The preview image shows an overview of all the slides on the canvas, exactly as they look when the entire Xplain is shown in the viewport. If no slides are defined then the preview shows a screenshot of the HelpXplain project canvas.