Drop one picture for a still GIF, or several for an animation. Put them in order, choose how long each shows and how often it loops, and the GIF is written on this device — up to 256 colours per frame, dithered if you like.
How the GIF is made
Every frame is fitted to the first picture's shape — padded or cropped, never stretched — and, by default, to at most 800 pixels on its long side, because GIF files grow fast. Each frame then gets its own palette of up to 256 colours, chosen by median cut from that frame's pixels, and optional Floyd–Steinberg dithering spreads the rounding error so gradients read as gradients rather than steps.
The colour reduction and the compression run in a background worker, so the page stays responsive. The finished GIF is read back by this site's own GIF reader — size and frame count checked — and opened by the browser before it is offered.
When GIF is the wrong format
GIF is old: 256 colours per frame and on-or-off transparency. A photograph saved as GIF bands and is usually several times larger than the JPEG it came from; the byte counts are shown side by side so you can see it. For a still photo, keep the JPEG, or use WebP or PNG. GIF is the right answer when the place you are posting expects a GIF — an animation for a chat, a forum or an old content system.