Nope, we aren’t allowed to use Pinterest or Google images, as was previously mentioned above, it’s incredibly hard to determine the original source of those images, and if they are copyrighted. Many of those sources are artists or companies.
Also, they will likely be approved as they are just images, unless watermarked episode has no way of determining where they came from. This is why you have to check that box when uploading, to say that you have the rights to use the image.
It is unlikely to effect your story being allowed on the app, however, should it be reported for using copyrighted images, then it would become an issue, and you are opening yourself up to possible legal consequences should it trace back to an original artist/company.