Image format for mask

What format can be used to make masks that takes least space? what I need is only 8 bit alpha channel (no color and nothing else). I need it for an iPhone application so everything that can easily be opened with UIImage will be good (list of them her开发者_运维知识库e).

Use a png. When compiling Xcode optimizes pngs for performance on iOS and for using with UIImage whereas it does not do the same for other formats, rendering any external optimizations futile except for extreme circumstances.