Drop a GIF. Every frame is shown in a strip with its timing; choose one, several or all, and each is written as JPEG or PNG — the frame exactly as the animation shows it at that moment.
Every frame, as it is shown
A browser hands a page only a GIF's first frame. This page reads the GIF itself — the colour tables, the timing, the transparency and how each frame replaces the last — and rebuilds every frame exactly as the animation displays it, so a frame that only changes a corner still comes out as the whole picture.
Long GIFs are listed up to 300 frames, to keep memory in check on a phone; the page says when it stopped.
JPEG or PNG
GIF holds at most 256 colours per frame. JPEG suits photographic GIFs and is searched to matched quality; PNG keeps the frame's pixels exactly and suits graphics and pixel art. A GIF with transparent areas going to JPEG waits for you to choose their colour.