The directive is:
.. image:: filename.type
Given that the pdflatex-builder will prefer a pdf and the html-builder an image-format, using the form:
.. image:: filename.*
allows builders to select their preferred format, if available.
As with other inclusions, path name is relative to the current directory or to the root directory:
.. image:: path/to/filename.type
or
.. image:: /path/to/filename.type
See the page Pulp case study for an example.