PNG

[Spodi]

When the PNG is loaded into memory, it's converted to a bitmap (I think). So the only thing this will really do is allow you to package a smaller client.

Right on the money. Though it may decrease loading time, depending on the implementation. It is actually not uncommon for files to be compressed so they can load faster. CPUs and RAM increase in speed at a much faster rate than HDDs do at reading, so if the compression is good and the decompression is very fast and purely in memory, it can take a shorter time to load completely. A common example of this is the usage of something like UPX on EXE files. Whether or not this applies to using PNG instead of BMP, at least with this implementation, I have no idea.