Targa dates to 1984 and refuses to die, because game engines and 3D packages standardised on it for textures and never had a reason to move. It keeps an alpha channel, optionally compresses runs of identical pixels, and is simple enough that every tool reads it.

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 TGA file above and choose afterwards.