Where are RIB files and frames saved ?
Those files are saved in the Render Farm Path defined in Tools > Options > Paths dialog, from the application window (default is /tmp on POSIX platforms). Whenever you click on the Render preview button, a directory named k3d-preview-NNN is created, where NNN is the number of the directory. There you’ll find RIB file in frame000001 sub-directory, as well as log files. Same thing for frame and animation rendering, but the directory name which will be respectively ‘k3d-render-frame-NNN’ and ‘k3d-animation-frame-NNN’.