I would image they are hard coded into the app and as such you can not change or add to.
Custom one can be located and applied via the File button, but I guess you know that.
You could customize the ribbon to add a gallery of custom textures and apply those to the chart.
You will find an example of a custom gallery
http://www.andypope.info/vba/ribboneditor.htm
http://www.rondebruin.nl/galleries.htm
Selection of a image from the gallery will allow you to run a routine.
This routine can either load the image file from disk or copy it via the clipboard.