This is problem similar to shortcut arrows. Notice that Shell is combining 32-bit and not 32-bit image. In such case, it is impossible to determine what result is (because non 32-bit image has zero alpha channel in 32-bit color space -> it will become transparent). It is possible to workaround this by forcing resulting alpha-channel white, however, if i'm to implement this, you'll lose alpha blending video card acceleration (i don't want to). It may be possible to work around this directly in Shell in future versions.