It's easiest just to do the splitting over two lines ...
DECLARE @variable VARCHAR(MAX)
SET @variable = 'CTO 247\SPFI\Images\OutImages\SET04012016\BATCH_07042016124801\PS_20160407190200550\98394'
SET @variable = SUBSTRING(@variable,CharIndex('PS_', @variable), LEN(@variable))
SELECT SUBSTRING(@variable, 0, CHARINDEX('\', @variable))