In zulip/zulip#16210, we're implementing thumbnailing for uploaded images.

We should use thumbnails in the mobile app. In particular, the message view should show thumbnails, while the lightbox should download the full-scale images.

Blockers:

• Generate thumbnails for uploaded images zulip#16210

Flutter issue:

• Make use of thumbnails zulip-flutter#799

#api design threads:

• https://chat.zulip.org/#narrow/stream/378-api-design/topic/thumbnailing.20transition.20for.209.2E0/near/1886620
• https://chat.zulip.org/#narrow/stream/378-api-design/topic/thumbnailing.20URLs/near/1843871
• https://chat.zulip.org/#narrow/stream/378-api-design/topic/thumbnail.20placeholder/near/1833796