Yes, you can use royalty free images for your ThemeForest item. But just make sure that you didn’t use it for clients download. I mean you can use it only for your theme preview. You can’t include it in your main download folder.
If there are some images or some other content which is not made by you then you have to provide the license for that item. Maybe it’s an image or plug-in or any other thing. If you are not able to provide that then please don’t include. Otherwise, they will reject your item.