You have to upload a valid .epub and valid .jpg. We check your cover and your ebook file to make sure that they are up to standards, alongside your copyright and metadata. You must also make sure that your content doesn’t fall into any of these groups. The validation process is both automatic and manual; if we find something wrong with your book, we will let you know via the platform where we explain you the issue. If you are not able to fix it yourself, please contact us via firstname.lastname@example.org for help.