Fix images in README with CRAN

[Jared Knowles]

From CRAN:

These have README.md referencing local images which do not get shipped
with the package, and hence missing in the README.html we generated from
the README.md for the CRAN package web pages. See the list of missing
image files below.

Can you pls fix to either ship the missing image files with the package
(in which case you could put them in a suitable subdirectory of 'tools'
in case things like 'vignettes' or 'man/figures' are not appropriate),
or use URLs (e.g. from your package source code repositories) instead?