Looking at openerp/osv/orm.py in _get_external_ids() we can see that the external ids are stored in "ir_model_data" and can be tied to the models using "ir_model_data"."model" and "ir_model_data"."res_id".  So to dump product external ids and images to a CSV:

copy (select "ir_model_data"."module" || '.' || "ir_model_data"."name" as "id", encode("product_product"."image",'base64') as "image" from "product_product" left join "ir_model_data" on (("ir_model_data"."model" = 'product.product') and ("product_product"."id" = "ir_model_data"."res_id")) where ("product_product"."image" is not null)) to '/tmp/product.product.images.csv' with csv header;

Or to pipe data from the source server to another server, connect from the remote server to the source database using psql:

\copy (select "ir_model_data"."module" || '.' || "ir_model_data"."name" as "id", encode("product_product"."image",'base64') as "image" from "product_product" left join "ir_model_data" on (("ir_model_data"."model" = 'product.product') and ("product_product"."id" = "ir_model_data"."res_id")) where ("product_product"."image" is not null)) to 'product.product.images.csv' with csv header;

Alternatively, I suppose it would be possible to:

• Export the database ids and the external ids using the Odoo web interface.
• Export the database ids and images directly from the database.
• Use a third-party tool and the database ids to associate the external ids with the images.
• Upload the images to Odoo.

But exporting the external id directly, as described above, would in many cases be preferable.

Another alternative would be to script oerplib to stream data from an old database to a new database.