Group avatars are an iMessage group feature. Set them through your line’s group metadata APIs when running on cloud or dedicated iMessage lines that support group management. For the sender avatar recipients see in a DM, configure your Skyline profile in the dashboard or CLI: 
sky skyline profile update --display-name "Acme Bot"
sky skyline avatar set --file ./avatar.png
Profile avatars apply across shared-pool routing so every user on your project sees a consistent sender identity.

​
Free plan scope

Group avatars and chat backgrounds are not included on the free plan. See iMessage provider for the full feature matrix.