Hi Heather, Thanks for contacting us! The issue you’ve run into is a current limitation of the History functionality (transparency isn’t supported). To preserve the transparency of your item, you need to re-upload it and drag it on from the “Get from Computer” list of uploads (as opposed to the “History” list in the Photo Basket). If you don’t have a copy on your computer to re-upload, you can open it from the Library > History tab and then re-save it to your computer.
Let us know if anything’s unclear. This limitation to items in History is something we’re aware of and will fix in the future.