Place an image which later gets replaced by the actual screenshot
Image fields are placeholder images with certain names. Netfabb replaces them with screenshots that are always of 1600×1200 pixels in size and 4:3 aspect ratio. Using a placeholder image of that aspect ratio helps, although it is technically not required as only the size and ratio of the embedded representation matters.

A 4:3 image for use as a placeholder in a report template. Feel free to download it through right-click and choosing Save image as, or whichever equivalent your web browser offers.