To avoid a foreach() error, I upgraded to 6.x-2.10rc1. This fixed the initial problem but I now get a SQL error on node delete:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AS r USING wysiwyg_imageupload_entity AS e WHERE e.iid=r.iid AND nid=4381' at line 1 query: DELETE FROM wysiwyg_imageupload_entity_revision AS r USING wysiwyg_imageupload_entity AS e WHERE e.iid=r.iid AND nid=4381 in ..../wysiwyg_imageupload/wysiwyg_imageupload.file.inc on line 207.

This query was changed from 6.x-2.9 to improve PostgreSQL compatibility as described in [989324]. However, it doesn't seem to work for my in MySQL. Changing the query on line 207 worked for me:

DELETE FROM r using {wysiwyg_imageupload_entity_revision} as r, {wysiwyg_imageupload_entity} AS e WHERE e.iid=r.iid AND nid=%d

My searches also turn up and alternative patch for the same line in [1239406]

DELETE FROM {wysiwyg_imageupload_entity_revision} WHERE iid IN (SELECT iid FROM {wysiwyg_imageupload_entity} WHERE nid=%d)

I don't use PostgreSQL and so cannot be sure that either or both of these changes would work across both databases. If someone is able to confirm the preferred approach, I'll prepare a patch relative to 2.10rc1