check that url actually served an image

oxdata/poster/models.py

@@ -79,6 +79,13 @@ class PosterCache(models.Model):
                 self.status = e.reason
                 self.failed = True
                 self.save()
+        if self.image:
+            try:
+                self.image.width
+            except TypeError:
+                self.failed = True
+                self.status = "invalid image"
+                self.image.delete()
         return self.image
 
 class Poster(models.Model):