If you want to make an LibO Gallery there are two options: 1. you draw the gallery item with shapes within LibO and move them to the gallery 2. you add an image file to the gallery If you draw an gallery item with shapes, the drawing was stored in an binary block. To edit it, you have to move the gallery item from the gallery to the drawing area, edit it delete the item from the gallery and add it again. It would be way easier (and without binary block), to save the drawing as .odg file in the gallery folder together with the images and by edit the .odg file, the gallery item was updated. So at the end you have .odg files (eat your own dogfood) which you preview in the gallery (code from the welcome page). By copy file to the correct folder, the user can edit the gallery. Label of the gallery item can be the group name of the gallery item (like you show in the navigator) or the filename, when no groupname was available. Pictures, Audio, Video can be stored native OR within an .odg file. So you have full flexibility and easy to maintain.
as there is for all galeries within LibO an .odg drawing available, I can offer to do the update of the gallery content within LibO.
You asked for this in bug 126934 and related in bug 127627 to "use .stg .stv .str files". In fact, the gallery is build pretty straightforward from graphics and even audio files in extra/source/gallery/. Don't see how this can be made simpler. *** This bug has been marked as a duplicate of bug 126934 ***