When I save it, luckly it stays as PNG.
If you still have copies of the overworld you sent me before, it's ok. It'll stay as a BMP until you upload it, so if you open it again and save it as a PNG, it won't change.
If it's easier, you can make it in paint. Although I'd like to request you save it as PNG instead of BMP, please! :) Or else it'll change to JPEG automatically when you upload it.