I deleted the configuration of Dolphin in ~/.kde4, and restarted.
I got into the situation that farcusnz describes: the information about folders (pictures+text) began popping up on the right-hand side of the Dolphin window, in a separate subwindow. (This reminded me the Lernaean Hydra.) It was possible to delete that subwindow, which solved the problem. Now, when the mouse pointer stands on the name of a folder of file, there is no popup window appearing.
I don’t have those settings in my Dolphin.
In “Settings”, I don’t have “View Modes”. I have “Configure Dolphin” in “Settings”, where I have “View Modes”. But the “View Modes” tab does not contain “Use Thumbnails” checkbox. In fact, I found nowhere the “Use Thumbnails” options.