Maybe you can fix this by changing the sync settings of your iOS device with OMV. Or you can install an extension that automatically copies photos from your gallery directly to the storage space built on OMV. I tried this method, and I succeeded. Moreover, I always find tips and tricks about pictures, such as storage, synchronization, copying on https://backlightblog.com/star-photography-iphone. It is specifically designed for information on Apple devices and is much easier to find centralized answers on topics.