Over half the questions posted to the mailing-list are centered around incorrect file and directory read/write permissions. The short version is that both your Webserver and Smokeping user need appropriate read-write permissions on the image, RRD and cache directories.

The FAQ page gets into this with detailed examples.

You can view your Smokeping directory permissions with this shell script.

We now have a list of third-party probes not included with Smokeping but which some might find useful or serve as examples to create your own.