At this point, there is no direct method to export from Map Layout to an image. It's something we're looking into due to recent requests. For the time being, we suggest either exporting to PDF and importing that (or using a tool like Preview or Graphic Converter to convert the PDF to an appropriate image format for your uses).
To convert with Preview, do the following:
- Choose File > Print and then choose PDF and Save As... to store the PDF file
- Open the PDF file in Preview
- Choose File > Save As...
- Select your file format and saving options and choose save
Unfortunately, there doesn't appear to be an Automator action from Preview to perform this function, so it will need to be done manually.