Xara3D is a vector program and will not produce 3D images from bitmaps, even if they are contained within WMF or EMF or .XAR files. If you create WMF/XAR files using the line/curve/freehand tools of almost all vector packages, the files will load into Xara3D and can be extruded. Bitmap images(JPEG/GIF/BMP/PNG files) can be imported into Xara3D for use as texture fills or backgrounds and can be applied as texture fills to simple 3D object buttons (circles, squares, etc).
If you need to extrude bitmap images you will first have to produce vector equivalents. This you can do by "hand" tracing the images using the freehand/curve/line tools of a vector drawing program such as Xara X, CorelDRAW, Illustrator or Xara Webster 2 (which is bundled with Xara3D 5 or later).