I took a pass last year at switching to PNG format instead of BMP format in TacOps. It turned into a nightmare real quick because so many of the Windows graphic API calls use bitmaps whose fields and data structures are the same or very similar to those of BMP format. The existing code needed so many on the fly conversions that the speed hit would have left too many older computers behind. A near total rewrite would have fixed most of the issues but I decided I had better things to do with two or three months of coding and testing time.
Or I could have solved most of the problems in a few weeks by moving to Apple Quicktime drawing routines but then the Windows users would have had to install Quicktime. I don't trust Apple Inc enough anymore to require that.
That being said, I hope to eventually try to switch to using PNG format for art files that are stored on hard disk.