I made a custom template that shows The title, an image of the download and a download button:
As you can see the first two thumbnails don’t have transparency on them, but the other images do. In the template I am using the shortcode: [thumb_150x150]
The featured images of all downloads are all PNG’s with transparency. Any idea how to fix this and why the first two images look like they have a flattened white alpha channel?
I have resolved the problem. It seems that when a featured images is not square i.e. 350 x 316px, the background is turned white. I tried resizing the images square to 316 x 316px and the problem was solved.