The "cast" screenshots were made with asciienema and then running through asciicast2gif.

For example:

asciinema rec trepan-xpy-assign.cast

You can edit the .cast files. The specific commands used after this were:

$
$ asciicast2gif -w 60 -h 16 trepan-xpy-assign.{cast,gif}
$ asciicast2gif -w 60 -h 16 trepan-xpy-ifelse.{cast,gif}