I've already written about how to get a graphic from a file on your computer into the Gallery. But how do you get a graphic you like, that's just sitting in some OpenOffice document, into the Gallery? You could just copy it and paste it from that document to others, but putting it in the Gallery is more convenient.
3. Select the theme, the category you want to put the graphic in, in the gallery.
6. Click and hold down, and keep holding down on the graphic, for at least two seconds. Don't release the mouse, and drag the graphic into the gallery.