You can set the avatar for a chat user via the "blob_id" field. You will need to upload a file into the Content Module and then connect it to the user with the mentioned "blob_id" field, as it performs as a link to the uploaded file in the Content Module.
Check our SDK documentation for further details:
iOS SDK - Users | iOS SDK | QuickBlox
Android SDK - Users | Android SDK | QuickBlox
React Native SDK - Users | React Native SDK | QuickBlox
Flutter SDK - Users | Flutter SDK | QuickBlox
Feel free to check our official documentation for more information that will help you to integrate QuickBlox services into your applications / projects or contact our Support team by submitting a ticket if you have any other questions.