Image import should consider transparency

[Artoria2e5]

At https://youtube.com/watch?v=VLZjd_Y1gJ8&t=8m33s, an image with transparency is turned into a solidly-filled image with a lot of… wood due to the transparent background being rgba(0, 0, 0, 0). The image import procedure should probably choose to just do nothing when the alpha value is less than 0.5.