JPEG is the format photographs live in. Its compression is lossy - it discards detail the eye is bad at noticing, and discards a little more every time the file is re-saved - which buys enormous size savings on photographic images and makes a mess of screenshots, logos and text. It has no transparency.
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 JPG file above and choose afterwards.