I realised that although I could upload Media fine WordPress would not insert images into the visual editor (nothing inserted; no HTML code added; no debug errors).
I did troubleshooting: disabled my custom theme and reverted to 2012 theme; reinstalled WP 3.5.1 and then deactivated and individually reactivating plugins. With just Simple Fields plugin running the problem was isolated.
I've seen possibly related problems with this plugin in the past (~1 year ago) but could not see more recent reports or solutions.
Could you help me solve this, please?