Use base64 encoded str for image data

src/exo/master/api.py

@@ -566,14 +566,10 @@ class API:
             )
             assert isinstance(chunk, ImageChunk)
 
-            # Convert image bytes to base64
-            import base64
-
-            b64_image = base64.b64encode(chunk.data).decode("utf-8")
-
+            # chunk.data is already base64-encoded string
             images.append(
                 ImageData(
-                    b64_json=b64_image
+                    b64_json=chunk.data
                     if payload.response_format == "b64_json"
                     else None,
                     url=None,  # URL format not implemented yet

src/exo/shared/types/chunks.py

@@ -23,7 +23,7 @@ class TokenChunk(BaseChunk):
 
 
 class ImageChunk(BaseChunk):
-    data: bytes
+    data: str  # Base64-encoded image data
     finish_reason: FinishReason | None = None