Once again...

GIF: A lossless 8-bit colour image compression format which supports transparency and animation. Best suited for solid fields of colour with hard boundaries, like comics.

JPEG: A variably lossy 24-bit colour image compression format. Best suited for photographs or art featuring gradients and/or soft transitions.

PNG: A lossless 24/32-bit colour image compression format, loosely analogous to a combination toaster/vacuum cleaner. Quite specifically not designed for printable graphic standards, and featuring non-standardized compression algorithms of wildly variable efficiency with a resultant file size typically 800% to 900% greater than a virtually identical JPEG. Best suited for artists who hate learning about the digital basics of their craft, fail to grasp the concept of keeping separate web and print-quality files, and who don't have to pay to keep the lights on.