<p>What is your favorite, lossless image format for games (namely 2d games)? And why?</p>

<p>Some things to take into consideration are size on disk, overhead for converting to a usable format, and features of the format (ie alpha support).</p>

<p>There is no best answer, but be sure to back yours up the best you can!</p>

I'd suggest PNG. Most software supports writing it, most libraries support reading it, it's lossless and supports alpha transparency. And it's a standard format.
And, maybe important for hobbyist 2D games, very small images also result in very small files (i.e. a 16x16 icon can be 1KB or less).