The picture fill function would be on the Text Fill menu rather than the Shape Fill menu if it was available, but it isn't there -- but only for the "new" WordArt version. Fortunately, you can get the old WordArt version by making a Word 97-2003 (*.doc)
file in Word 2013.
You can either save your current document as a *.doc file, or make a new blank document and save that as a *.doc file. Then the Insert > WordArt button will give you the old WordArt functionality, including the picture fill. If you've saved the current document
as *.doc, you can then click File > Convert and save (which automatically makes it a *.docx again). If you've made a new blank *.doc and created the WordArt there, you can copy/paste it into your *.docx and it will continue to be old-style WordArt.