After merging (or checking out locally) you can go to /meta/twitter-card-generator/ to see a list of cards showing up. You may edit the _layout file for different descriptions (there's a JSON object), and to change the visuals (e.g. the SVG).
When you are done, you can just screenshot the cards and save them as a .png image into /assets/zurich/social/ under the same name as the speaker image is. I usually just use Firefox for this, in the devtools you can just right-click a card and click "Screenshot node", which saves a .png file of the correct size automatically into your "Downloads" folder.
Note that these files are recompressed by Twitter as .jpegs, so there is no harm in having large files.
The Twitter Card Validator service can be used for checking metadata (it also works on this page, it will show Ashley's card that is embedded in the metadata in ), but it only works for already-on-line pages, doesn't work with locally served devpages (as it uses Twitter's crawler).