Disable optipng

Rel: #633

.image_optim.yml

@@ -1,6 +1,7 @@
 verbose: false
 skip_missing_workers: true
 allow_lossy: true
+threads: 1
 advpng: false
 gifsicle:
   interlace: false
@@ -12,10 +13,7 @@ jpegoptim:
   max_quality: 100
 jpegrecompress: false
 jpegtran: false
-optipng:
-  level: 3
-  interlace: false
-  strip: true
+optipng: false
 pngcrush: false
 pngout: false
 pngquant: