Try converting back to varchar, rather than nvarchar? If your initial string was a varchar, you need to convert back to that. For example:
SELECT CONVERT(nvarchar(100),CONVERT(varbinary,CONVERT(image,'sdfashgu3o4ggjkxchbvkjasgvftkj'))); --varchar to nvarchar
SELECT CONVERT(varchar(100),CONVERT(varbinary,CONVERT(image,'sdfashgu3o4ggjkxchbvkjasgvftkj'))); --varchar to varchar
Also, at least on SQL Server 2012 SP4, you can't CONVERT an nvarchar to image, thus this seems to imply more so that your original data type was varchar.
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does :-P
Please always remember to encapsulate your code in IFCode Markup. For example [code=sql] [/code]
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