Worth a try, but in V6 at least, exporting and replacing a previous html file will recreate new image content in the html_files folder (based on what's visible in the Xara document) and delete all old image data from the folder--providing it was previously generated by Xara. In other words, no old image content in that folder should remain after the new export unless it had manually been put there by the user, seperately from Xara. Also, Xara won't delete files that it placed in there when the user assigns that file to a button link in Web Properties, if the user later chooses to break the link.

I used to be concerned about flushing the folder/its content to avoid clutter being uploaded before re-exporting, but I'm now convinced it's not necessary (although it was a hard habit to break). In fact, it is potentially harmful to delete the folder completely now as Xara only puts linked files in there at attachment time, not export time (to gaurantee a copy gets there), so they won't reappear on subsequent export if inadvertently deleted when you delete the folder.