This is correct:
But the question was about increasing column length...

SQL> desc images
Name Null? Type
----------------------------------------------------- -------- --------------------
IMAGE LONG RAW
DB VARCHAR2(100)
FIELD VARCHAR2(10)
IMGKEY NOT NULL NUMBER(20)
MAINKEY VARCHAR2(125)

SQL> alter table images modify db varchar2(80);
alter table images modify db varchar2(80)
*
ERROR at line 1:
ORA-01441: column to be modified must be empty to decrease column length

SQL> alter table images modify db varchar(120);

Table altered.

SQL> desc images
Name Null? Type
----------------------------------------------------- -------- -----------------------------
IMAGE LONG RAW
DB VARCHAR2(120)
FIELD VARCHAR2(10)
IMGKEY NOT NULL NUMBER(20)
MAINKEY VARCHAR2(125)

Cheers!