- Sprite with invalid mask
For correctly displaying your sprite, you'll have to use the mask only in gray grade colors, or black and white:
- Sprite with valid mask
You'll have to fix the mask for this bug to be fixed. I've already written how to do that here:
How to: Fixing lazily-made masks automatically with LazyFixTool
How to: Fixing lazily-made masks manually with GIMP
How to: Fixing lazily-made masks manually with Paint.NET
Another example:
Wrong mask:
Correct mask: