[Admins]

v3 supports 24 bit pngs and jpegs and bmps (pngs are best for tilesets). Soon there will be v3.1, more people will use it and you can possibly restrict your server to only let v3.1 on your server or set different tilesets depending no the version (setting a stripped 256 colors version as tileset on v2.2, and a full 24 bit tileset on v3).

Another possibility is to divide the tilset into several images, depending on the purpose of the tiles. Each of those images then can have their own 256 color palettes. By dividing the tileset you also have the advantage that there is less data to download when you change a single tile. A full tileset is often >300 k, divided in several images each image can be have 20k-30k which is ok I guess. That techinque should be used when using 24bit tilesets too.