Let me first correct the terms uses: "gallery" refers to the Zenphoto site in total, you are referring to a single "album".
> My experience with php is limited so a piece of example code would be greatly appreciated
Sorry, then you will not really far here. There is no simple code for this as this is off the standard.
First you should read the theming tutorial:
https://www.zenphoto.org/news/theming-tutorial/
To achieve this you first need to create a custom `index.php` file of your theme.
Then you have to manually set the album you want to display to "current" there. There is a function name `makeAlbumCurrent()` do to so. Then you need to add the `next_image` loop. Note I have never tried/tested this, this is how it theoretically might work.Also note that his has of course impact on the gallery in total as you will literally will "disable" all other top level albums by doing this.
In any case the pagination will not work as expected as the the toplevel gallery URLs are `pathtoyourinsall/page/1` and those of an album are `pathtoyourinstall/albumname/page/1`. So all further pages will change to that album based URL. That is how Zenphoto works.