Good question 
Monzo crowdsource the merchant’s logos & other data from Monzo users, this blog explains how that works -
https://monzo.com/blog/2016/04/11/crowdsourcing-merchant-data/
Android users can’t submit updates through the app yet but they can message the support team with them, via the in-app chat instead.
Monzo also pulls data from Foursquare & Google to help identify and build their database you can see some of the data they retrieve via the API -
