So you're using gistup to create gists (and blocks) from the command line. If not, see this tutorial for a quick overview.
And now you want to add thumbnails to your gists so that your blocks portfolio looks all pretty, right. How?
To have a thumbnail associated with your gist you need to create a 230x120
pixel PNG image file named thumbnail.png and include this at the top-level of
your gist's repo (generated by gistup).
The main trick is in finding an appropriate image and then right-sizing it for
the 230x120 dimensions that the blocks site expects your thumbnail.png to
