Use asterisk instead of double slash
DECLARE @x TABLE(item XML)
DECLARE @schemaname VARCHAR(100)
SET @schemaname = 'GaleriesSchem3'
INSERT into @x
SELECT '
<GaleriesSchem2>
<Image_1 OriginalName="Image">4814111.jpg</Image_1>
<Image_2 OriginalName="Image2">481411.jpg</Image_2>
</GaleriesSchem2>
<GaleriesSchem3>
<Image_1 OriginalName="Image">4814111_3.jpg</Image_1>
<Image_2 OriginalName="Image2">481411_3.jpg</Image_2>
</GaleriesSchem3>
'
SELECT rref.value('.', 'varchar(MAX)') AS 'Value'
FROM @x
CROSS APPLY
item.nodes('*[local-name()=sql:variable("@schemaname")]/node()') AS Results(rref)
See demo on SQLFiddle