Different publishers have different criteria for your book covers. We use the criteria that KDP requires. When we are working with an author, we point out that when using the Kindle Direct Publishing platform, you have access to templates that come with pictures or you can provide your own. If you are using the proper dimensions, the picture will fit within their templates forming a great cover.
watch this blog for some useful hints about creating your own Media Kit. And don’t dismiss the topic. If you plan on having tv/radio interviews or plan on placing your book in a bookstore, then a Media Kit is a must.