Using "Picture Function" to create animated arrows and progress in a GUI.

I saw this post: https://mcreator.net/tracker/issue/66028, which showed the "New Pictures Function" from 2020.5 being used to create an arrow or progress bar for a block GUI.

I have no idea what the person in that post meant by "Pictures Function", or how to use it and create an animated GUI element. I know that now you can make different images appear at different times, but i have no idea how to link that to make a progress bar.