The error you're getting is nothing to do with the picture (which you're not actually saving at the moment)
It means one of the columns in your table doesn't match the type of data you're trying to insert into it.
Start by specifying an explicit column list in your insert statement:
INSERT INTO Road_Sign (sign_id, year_made, route, section, date_intalled, expiry_date, position, condition, chainage_from, chainage_to, picture) VALUES (@sign_id, @year_made, @route, @section, @date_intalled, @expiry_date, @position, @condition, @chainage_from, @chainage_to, @picture)
If it still doesn't work, compare the column types in your table to the parameter types you're passing in.