Hey @knittyrobin52! All of our landing page templates are fully responsive and each template is made to be as dynamic as possible, so it should adjust to the width and height of the uploaded image. As a result, the exact dimensions shouldn't matter as much as the crop/aspect ratio (in terms of how it'll look on different devices). The best way to test is usually to try uploading it and then view the landing page on different screen sizes (or resize your browser window) to see how it scales.
That is to say, we don't have a list with exact dimensions that we can point you to at the moment; however, if you are struggling with a placeholder in particular template, let us know the name of the template/which image it is and I can see if we can provide some additional guidance! :)