set as property rather than attribute
diff --git a/coders/webp.c b/coders/webp.c
index f0ed7f2..9a376d5 100644
--- a/coders/webp.c
+++ b/coders/webp.c
@@ -425,45 +425,47 @@
   data.bytes=stream;
   data.size=length;
   demux=WebPDemux(&data);
-  if (WebPDemuxGetFrame(demux,1,&iter)) {
-    do {
-      if (image_count != 0)
-        {
-          AcquireNextImage(image_info,image,exception);
-          if (GetNextImageInList(image) == (Image *) NULL)
-            break;
-          image=SyncNextImageInList(image);
-          CloneImageProperties(image,original_image);
-          image->page.x=iter.x_offset;
-          image->page.y=iter.y_offset;
-          webp_status=ReadSingleWEBPImage(image,iter.fragment.bytes,
-            iter.fragment.size,configure,exception,MagickFalse);
-        }
-      else
-        {
-          image->page.x=iter.x_offset;
-          image->page.y=iter.y_offset;
-          webp_status=ReadSingleWEBPImage(image,iter.fragment.bytes,
-            iter.fragment.size,configure,exception,MagickTrue);
-        }
-      if (webp_status != VP8_STATUS_OK)
-        break;
-
-      image->page.width=canvas_width;
-      image->page.height=canvas_height;
-      image->ticks_per_second=100;
-      image->delay=iter.duration/10;
-      image->dispose=NoneDispose;
-      if (iter.dispose_method == WEBP_MUX_DISPOSE_BACKGROUND)
-        image->dispose=BackgroundDispose;
-      (void) SetImageArtifact(image,"webp:mux-blend","AtopPreviousAlphaBlend");
-      if (iter.blend_method == WEBP_MUX_BLEND)
-        (void) SetImageArtifact(image,"webp:mux-blend",
-          "AtopBackgroundAlphaBlend");
-      image_count++;
-    } while (WebPDemuxNextFrame(&iter));
-    WebPDemuxReleaseIterator(&iter);
-  }
+  if (WebPDemuxGetFrame(demux,1,&iter))
+    {
+      do
+      {
+        if (image_count != 0)
+          {
+            AcquireNextImage(image_info,image,exception);
+            if (GetNextImageInList(image) == (Image *) NULL)
+              break;
+            image=SyncNextImageInList(image);
+            CloneImageProperties(image,original_image);
+            image->page.x=iter.x_offset;
+            image->page.y=iter.y_offset;
+            webp_status=ReadSingleWEBPImage(image,iter.fragment.bytes,
+              iter.fragment.size,configure,exception,MagickFalse);
+          }
+        else
+          {
+            image->page.x=iter.x_offset;
+            image->page.y=iter.y_offset;
+            webp_status=ReadSingleWEBPImage(image,iter.fragment.bytes,
+              iter.fragment.size,configure,exception,MagickTrue);
+          }
+        if (webp_status != VP8_STATUS_OK)
+          break;
+        image->page.width=canvas_width;
+        image->page.height=canvas_height;
+        image->ticks_per_second=100;
+        image->delay=iter.duration/10;
+        image->dispose=NoneDispose;
+        if (iter.dispose_method == WEBP_MUX_DISPOSE_BACKGROUND)
+          image->dispose=BackgroundDispose;
+        (void) SetImageProperty(image,"webp:mux-blend",
+          "AtopPreviousAlphaBlend",exception);
+        if (iter.blend_method == WEBP_MUX_BLEND)
+          (void) SetImageProperty(image,"webp:mux-blend",
+            "AtopBackgroundAlphaBlend",exception);
+        image_count++;
+      } while (WebPDemuxNextFrame(&iter));
+      WebPDemuxReleaseIterator(&iter);
+    }
   WebPDemuxDelete(demux);
   return(webp_status);
 }