Do this first, and verify that the rows selected are the ones that you want to change:
SELECT U_Username,U_Picture,U_PicWidth,U_PicHeight
FROM w3t_Users
WHERE U_Picture is not null and U_Picture <> '' and U_Picture <> 'http://'
Then update them:
UPDATE w3t_Users
SET U_PicWidth = 48, U_PicHeight = 48
WHERE (U_Picture is not null and U_Picture <> '' and U_Picture <> 'http://')
Disclaimer: backup the database first.
