The Windows bitmap, which stores pixels raw with no compression worth mentioning. Files are enormous. It survives because it's trivial to read - if you're writing an image loader from scratch or feeding an embedded display, BMP is the format that needs no library.

Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a BMP file above and choose afterwards.