diff --git a/vllm/multimodal/utils.py b/vllm/multimodal/utils.py
index e55b8bbfdeaac..8691a61343ab6 100644
--- a/vllm/multimodal/utils.py
+++ b/vllm/multimodal/utils.py
@@ -35,8 +35,12 @@ def _load_image_from_data_url(image_url: str):
     return load_image_from_base64(image_base64)
 
 
-def fetch_image(image_url: str) -> Image.Image:
-    """Load PIL image from a url or base64 encoded openai GPT4V format"""
+def fetch_image(image_url: str, *, image_mode: str = "RGB") -> Image.Image:
+    """
+    Load a PIL image from a HTTP or base64 data URL.
+
+    By default, the image is converted into RGB format.
+    """
     if image_url.startswith('http'):
         _validate_remote_url(image_url, name="image_url")
 
@@ -53,7 +57,7 @@ def fetch_image(image_url: str) -> Image.Image:
         raise ValueError("Invalid 'image_url': A valid 'image_url' must start "
                          "with either 'data:image' or 'http'.")
 
-    return image
+    return image.convert(image_mode)
 
 
 class ImageFetchAiohttp:
@@ -70,8 +74,17 @@ def get_aiohttp_client(cls) -> aiohttp.ClientSession:
         return cls.aiohttp_client
 
     @classmethod
-    async def fetch_image(cls, image_url: str) -> Image.Image:
-        """Load PIL image from a url or base64 encoded openai GPT4V format"""
+    async def fetch_image(
+        cls,
+        image_url: str,
+        *,
+        image_mode: str = "RGB",
+    ) -> Image.Image:
+        """
+        Asynchronously load a PIL image from a HTTP or base64 data URL.
+
+        By default, the image is converted into RGB format.
+        """
 
         if image_url.startswith('http'):
             _validate_remote_url(image_url, name="image_url")
@@ -91,7 +104,7 @@ async def fetch_image(cls, image_url: str) -> Image.Image:
                 "Invalid 'image_url': A valid 'image_url' must start "
                 "with either 'data:image' or 'http'.")
 
-        return image
+        return image.convert(image_mode)
 
 
 async def async_get_and_parse_image(image_url: str) -> MultiModalDataDict:
@@ -99,12 +112,19 @@ async def async_get_and_parse_image(image_url: str) -> MultiModalDataDict:
     return {"image": image}
 
 
-def encode_image_base64(image: Image.Image, format: str = 'JPEG') -> str:
-    """Encode a pillow image to base64 format."""
+def encode_image_base64(
+    image: Image.Image,
+    *,
+    image_mode: str = "RGB",
+    format: str = "JPEG",
+) -> str:
+    """
+    Encode a pillow image to base64 format.
 
+    By default, the image is converted into RGB format before being encoded.
+    """
     buffered = BytesIO()
-    if format == 'JPEG':
-        image = image.convert('RGB')
+    image = image.convert(image_mode)
     image.save(buffered, format)
     return base64.b64encode(buffered.getvalue()).decode('utf-8')