UPDATE: Beeper, through their Twitter account, is saying that they’re working on it and “hope to have good news to share soon”:
Work continues to fix the issue causing the Beeper Mini outage. We know how hard this has been for those who loved using Beeper Mini, and we’re extremely sorry for the inconvenience. We are feeling good, though, and hope to have good news to share soon.
When iMessage user sends messages, videos, pictures, gifs to another iMessage user, it sends over via data. Similar to Signal, Telegram, Whatsapp, FB Messenger, etc.
When an iMessage user sends to an Android user, it sends it via SMS/MMS which is very limited.