Filter for image orientation?

Is it possible to filter for whether an image file is landscape or portrait, or work this out some other way such as by comparing length and width? My use case: my view shows a random image from a user's nodes in a sidebar block, and if the image is landscape it breaks the sidebar boundary, so I only want to show portrait images in that place. I've tried getting round this by using an image style which autorotates and crops the image to the sidebar width, but that makes landscape images look a bit rubbish, hence I'd prefer to filter them out altogether.