Similar to the changes made for the link plugin http://discourse.farcrycore.org/t/closed-insert-edit-link-fails-in-tinymce/404 the timeout in image_farcry/plugin.min.js needs to be increased to 10000 (setting is on line 51).
Thanks Mark, easy one. FC-3025, fixed in p700, p710, p720
Thanks Justin. Annoyingly turns out not to be the source of my new issue but at least it is removed from the list of possible causes