Transparent icons not used when available

I have an icon containing several images as follows:
16 x 16, 16 colors
16 x 16, 256 colors
16 x 16, true color (32-bit)
32 x 32, 16 colors
32 x 32, 256 colors
32 x 32, true color (32-bit)
48 x 48, 16 colors
48 x 48, 256 colors
48 x 48, true color (32-bit)

When I set it as the icon for a menu button TLB appears to be using the 32 x 32, 256 colors representation and not the 32 x 32, true color (32-bit) image.