The reason you can't find info about how to convert a panel to a button is because there is no magical/quick way to do this.

If you have placed the images in the gallery as bitmaps, you will have to open each panel that is created after dragging an image onto a page and save the png/bmp to a file on your hard disk. Once you have done this, you can reload the pngs/bmps you saved previously into buttons.

You can store either buttons or images in the gallery but be advised that if you want to use the images on the 'Images' tab of the gallery in buttons, you will have to follow the above procedure.

If you have a button that you'd like to insert into the gallery, simply open the gallery, select the 'Buttons' tab, then click 'Insert'.