Yeah you'll want to use a 
Byte[] type on the object. For the hibernate mapping the 
Byte[] datatype works for small images but it will cut off large images and files. You'll need to use 
BinaryBlob from the 
Large Object Mapping Types to store larger files.
So you'll probably want to use the 
blob datatype for the postgresql column, 
Byte[] field on the object and set 
BinaryBlob on the NH property. 
Ignoring the technical side of things I have always found it's better to store images and other files directly on the filesystem. It makes backup easier, it's faster, and the files are directly available to other systems.