An announcement board on your wallboards is a dashboard with an image. To edit the image:
Make sure that the image you wish to upload to your wallboard is in PNG or JPEG format. If your image is in PDF, take a screenshot of the image and save it in the correct format
Navigate to the wallboard you would like to edit and open the slide containing the announcement
Click on the pencil in the top right hand corner of the image and upload a new file from your computer
That's it - after the file has been uploaded it will automatically appear on the slide.