Current plan is to remove FreeImage support from the D3D build of Steem SSE to make it thinner.
This build would support: BMP, JPG, PNG, NEO.
With FreeImage, you have more choices (TIFF, PPM...) and quality options for JPG.
The DirectDraw build would still support FreeImage.
My guess is that PNG should be the preferred format, but one always gets surprised by what people use.