What is the maximum image size for a gallery?
When I upload photos (6000x4000) they don't show on my site I just get placeholders, no thumbnails. When I reduce the image size they show fine.
Is it zenphoto at fault or possibly my PHP settings?

Thanks in advance.

It's your server configuration and power that is not able to handle that large images. Please see:
https://www.zenphoto.org/news/problems-with-albums-and-images/