It's best to use either an FTP uploader software (like leech/smartftp/filezilla) or use your windows/apple explorer windows.
I don't think Xara will upload stand alone files like that.
Then in your code (website→head) put the complete path (<meta property="og:image" content="http://www.monkeypants.com.au/mpmthumb.jpg" /> ) including the extension, and that should do it.
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