@Qole, Rebhana: in the URI tab on my device all image-something mimetypes. including image-jpeg, are associated with hildon-browser.desktop:X-Osso-URI-Action-Open.
Can you confirm please? In order to make my hack work I have to change that to dbus-switchboard.

Also, if I apt-get purge dbus-switchboard, things get normal again and jpg get opened by the image-viewer. So it must be dbus-switchboard that directs them to the browser.