How do I undo "always show pictures from this sender" on Android gmail? I accidentally clicked 'Always Display Images from This Sender' ('Always show pictures from this sender' in 2020) and was wondering if there's a way to undo this setting in Android gmail.

It's straightforward to undo on web Gmail, but not the same with Android (that I know of). Any help is appreciated!
In the Android Gmail app,

From the main inbox or inside any email: Bring up the Menu > Settings > General Settings. Now click the Menu button again to bring up sub-menu of options > Clear picture approvals.

WARNING: This clears ALL your approved sender settings, I believe there is no option to revoke by individual email senders.