Currently, the image uploading stopped working because Telegraph X app is based on
Telegra.ph API that is supported by the Telegram team.
Yesterday, Pavel Durov
announced disabling new media uploads for this platform:
✂️ We’ve also disabled new media uploads to Telegraph, our standalone blogging tool, which seems to have been misused by anonymous actors.
Since the Telegraph X closely depends on
Telegra.ph API the image uploading will be removed from the app as well soon.
Alternatively, you can still paste image links to your telegraph posts.