Several users reported that corrupted thumbnails were being generated from PNG24 uploads with a transparency channel, with the issue manifesting itself as black/faintly colored bars horizontally across the image at random positions.
The issue has been identified in how the thumbnail server's code deals with alpha channel. Instead of compositing the resized copy of the original image over a blank white canvas the updated code now removes it as per www.imagemagick.org: Masks -- IM v6 Examples
Thumbnail cache of all PNG uploads since Feb, 26 this year has been removed to allow for new thumbnails to be generated in their place, which should no longer exhibit image corruption.
The issue has been identified in how the thumbnail server's code deals with alpha channel. Instead of compositing the resized copy of the original image over a blank white canvas the updated code now removes it as per www.imagemagick.org: Masks -- IM v6 Examples
Thumbnail cache of all PNG uploads since Feb, 26 this year has been removed to allow for new thumbnails to be generated in their place, which should no longer exhibit image corruption.