In Word 2003 WordArt, if you use the Format WordArt dialog, Colors and Lines tab, Color | Fill Effects, there is a Picture tab in the Fill Effects dialog. You can choose any picture to fill the WordArt.
In Word 2007, you can click the Shape Fill button in the WordArt Styles group on the WordArt Tools | Format tab and select Picture. You can also get to the classic Format WordArt dialog by clicking the dialog launcher Size group, then select the Colors and
Lines tab and click Fill Effects..., which gives you the whole selection.
In Word 2010, in a .doc file, you get the same WordArt tools as in Word 2007. In a .docx, you get the same Picture option on the Shape Fill menu, but it fills the WordArt bounding box, not the WordArt text. To fill the actual text, you have to use the Text
Fill button in the WordArt Styles group, and it includes gradients but not Picture (and you have to actually select the text, not just click in it, since each letter is individual--not a single object as in previous versions).