T-SQL:
update TBL
set picture = Replace(picture, 'servera', 'serverb')
where picture like '%servera%'
Oracle:
update TBL
set picture = replace(picture, 'servera', 'serverb')
where picture like '%servera%'
MySQL:
update TBL
set picture = REPLACE(picture, 'servera', 'serverb')
where picture like '%servera%'