Hm, it works for me quite straighforward. I have a table with two columns: ImageID: numeric and Image: image. Also I have a ByteStore class with two fields int bytesID and byte[] bytes.
The SQLMap is trivial, and it works (I use it to store uploaded images, )
<insert id='insertByteStore' parameterClass='.....ByteStore'>
insert into Images (ImageID,image)
values ( #bytesID#,#bytes#)
</insert>
