The newest version of Web Designer 6 and and Xara Designer Pro 6 has an undocumented feature allowing you to name specific images via the 'names' dialogue.

Right-click your image and select 'Names'
Enter filename="imagename" (do not give it a .jpg or .png extension, Xara will do this automatically based on the image type at export)

Click image for larger version.  Name: image-file-name.jpg  Views: 1281  Size: 88.3 KB  ID: 74683

Exported file and html result:
<img class="xr_ap" src="index_htm_files/starfish.jpg"

!WARNING!: Use with care - this is an undocumented feature and not officialy support by Xara. Not for use with Highslide popups or galleries.