Update README to use local preview image (#481)

README.md

@@ -2,7 +2,7 @@
 
 Your personal AI assistant that remembers everything across **Telegram, Slack, Discord, WhatsApp, and Signal**. Powered by the [Letta Code SDK](https://github.com/letta-ai/letta-code-sdk).
 
-<img width="750" alt="lettabot-preview" src="https://github.com/user-attachments/assets/9f01b845-d5b0-447b-927d-ae15f9ec7511" />
+<img width="750" alt="lettabot-preview" src="assets/preview.jpg" />
 
 [![Deploy on Railway](https://railway.com/button.svg)](https://railway.com/deploy/lettabot?utm_medium=integration&utm_source=template&utm_campaign=generic)