Are ebooks that are primarily photos frowned upon, or are they ok?
There’s nothing wrong with picture books, but you need to be careful how you format the captions – if you embed them into the image they can be difficult to read, and if you separate them then you need to be mindful of the reader’s experience across different devices.
If you format by hand following the KF8 guidelines you can actually do some really cool things, but you need to know some advanced HTML tactics. For example, you can include clickable text that appears in the right place in the image but which gets larger and appears in a popover if you click on it (kindle fire only), or format a full comic book on the kindle.
The other problem you’ll run into with large images is that you need to make sure that they are optimized so that your files aren’t too large, otherwise your profits will get eaten away by the delivery costs.