I created NasPics specifically for this reason. I wrote it in pure Go with no dependencies for ease of cross compilation since my NAS runs a weird old Debian based distribution of Linux on ARM. Running it is just cross compiling a binary, copying over and executing. For restart across boots, I just added a simple SystemD service file.

https://github.com/srinathh/naspics

(edit: some data destination paths are probably hard coded since I wrote it for myself but can be easily broken out into command line options if needed)