improve warning message for missing image extension (#7150)

Author: pmeier

torchvision/io/image.py

@@ -10,7 +10,12 @@
 try:
     _load_library("image")
 except (ImportError, OSError) as e:
-    warn(f"Failed to load image Python extension: {e}")
+    warn(
+        f"Failed to load image Python extension: '{e}'"
+        f"If you don't plan on using image functionality from `torchvision.io`, you can ignore this warning. "
+        f"Otherwise, there might be something wrong with your environment. "
+        f"Did you have `libjpeg` or `libpng` installed before building `torchvision` from source?"
+    )
 
 
 class ImageReadMode(Enum):