image edits not saved

I have to test this some more, but am experiencing the following strange issue:

- Using IMCE + TinyMCE. Works fine for nodes without Version or OG enabled.
- For nodes in a Group, with Version turned on, the first time the user edits the node and makes _just_ an image update (adds/deletes an image using IMCE).
-- The node is saved, but looking at the Diff, it shows 'identical'
-- Re-editing the node and re-doing the update works.
-- If the user makes and image + text change first time round, that update is saved.
- Nothing in the logs

Any ideas as to what could cause this?