I think the problem of images that aren't relevant to content is universal - lots of people not using Xara software will add images for presentation purposes that aren't content related (brings to mind the old 1-pixel image trick of old). Xara does generally encourage more images to be created - either automatically (as you have pointed out) or for aesthetic reasons - so I don't think Xara generated sites will particularly upset search engines, because plenty of sites not generated by xara have presentational images without alt tags.