There really is no other answer Dan.
As you're sure to have read in the other numerous threads regarding the same phenomenon, the cause is due to new edits/changes (which changes the numerical sorting order of exported images) and re uploads.
My only suggestion to you would be to delete the files from the server before you re publish.
Of course this means you must use an FTP client rather than relying on Xara's built-in one.
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