diff --git a/lib/image_formats.py b/lib/image_formats.py
index c004ce0..047000b 100644
--- a/lib/image_formats.py
+++ b/lib/image_formats.py
@@ -6,15 +6,14 @@
 
 
 ImageFormat = collections.namedtuple("ImageFormat", ["extension", "formatter"])
+DEFAULT_IMAGE_FORMAT = "png"
 
 
 def pick_image_format():
-    try:
-        fmt = os.environ["IMAGE_FORMAT"]
-    except KeyError:
-        raise Exception("You must set the IMAGE_FORMAT environment variable to svg or png")
+    fmt = os.environ.get("IMAGE_FORMAT", DEFAULT_IMAGE_FORMAT)
 
     if fmt == "svg":
         return ImageFormat("svg", SVG)
     if fmt == "png":
         return ImageFormat("png", Image)
+    raise ValueError(f"Unknown IMAGE_FORMAT '{fmt}', you must specify one of 'svg' or 'png'")