As confirmed by SQLDeveloper development team, Here is some information on this:
When migrating SQL Server/Sybase IMAGES to Oracle in 'offline' mode, we cant upload the data using SQL Loader directly into the BLOB column because:
1. IMAGEs in SQL Server/Sybase dumped out in HEX.
2. Oracle s HEXTORAW function can only take 4000 chars
3. So data gets uploaded in chunks against the CLOB
4. Then a procedure is run with HEXTORAW to convert it to BLOB
5. After the data load is over, this additional CLOB column can be dropped from the table as Oracle allows you to drop a column.
All this happens in automated fashion for offline data load process. For On-line data move, this is not required.