diff --git a/src/guidellm/backend/openai.py b/src/guidellm/backend/openai.py
index f75bb3b..90d2791 100644
--- a/src/guidellm/backend/openai.py
+++ b/src/guidellm/backend/openai.py
@@ -179,7 +179,7 @@ def _build_messages(self, request: TextGenerationRequest) -> Dict:
                 stream = io.BytesIO()
                 im_format = image.image.format or "PNG"
                 image.image.save(stream, format=im_format)
-                im_b64 = base64.b64encode(stream.getvalue()).decode("ascii")
+                im_b64 = base64.b64encode(stream.getvalue()).decode("utf-8")
                 image_url = {"url": f"data:image/{im_format.lower()};base64,{im_b64}"}
                 content.append({"type": "image_url", "image_url": image_url})
 
diff --git a/src/guidellm/utils/images.py b/src/guidellm/utils/images.py
index 5c5a727..5d73bc0 100644
--- a/src/guidellm/utils/images.py
+++ b/src/guidellm/utils/images.py
@@ -65,4 +65,4 @@ def load_images(data: str) -> List[ImageDescriptor]:
                     )
                 )
 
-    return images
+        return images