This post is almost a note to myself but I hope to help others when you Google for this little problem.
One of the features introduced with Oracle Apex 3.1 is the Declarative BLOB support. About this subject there is already a good article on Oracle By Example (OBE) site: Defining and Viewing BLOB Data in Oracle Application Express 3.1.
Last Friday I lost almost an hour just to have a simple classic report with an image column. I was getting a ORA-01403 error because simply I wasn't referencing the Table Primary Key in my SQL Query!
These are the steps needed to have the image column successfully displayed:
Report Source Query
Important Tip: You must include the Primary Key column in your query.
Report Column Attributes
Format Mask Attributes
Important Tip: Database table and column names are case sensitive.
Tabular Form Element
Include the reference to the Schema, Table and Blob Column.