The documentation says:

It is normal that images from directories inside your given gallery directory do not appear. BG only displays images in the given directory. If you want to display the other galleries (sets of images in nested directories), you have to use a [bg|...] tag for each of them (or join the images, separated by using prefixes, for example).

I am dealing with an array of folder references of multiple galleries, but I'm trying to combine them so they don't break into separate chunks on my page. Does anyone have any clue what the phrase "join the images, separated by using prefixes" means?