Make an animated GIF from images

Because GIF stores at most 256 colours per frame, and a photograph contains far more than that. The encoder picks the 256 that best represent each frame, which is as good as the format allows, but smooth gradients - a sky, a shadow - will show banding. That is the format rather than the tool: for a photograph, an animated WebP or a short video is a far better container. GIF is at its best on flat colour, which is why screen recordings and line art come out looking identical to the source.

GIF stores frame delays in hundredths of a second, so the finest step it can express is 10 ms and a 16.7 ms frame becomes 20 ms. There is no way around it inside the format. The page reports the delay it actually wrote rather than the one you typed, because if you are matching a source frame rate you need to know which of the two you got.

A GIF has one canvas, so they have to be reconciled. The first image sets the size, and the rest either fit inside it with a white border or fill it and lose their edges - you choose which. Nothing is stretched, because a stretched frame in the middle of an animation is the most obvious possible defect and is never what anyone wanted.

Yes, but only as one bit: a pixel is either fully transparent or fully opaque, because that is all GIF stores. A soft, antialiased edge has to fall on one side or the other, and the cut is made at half opacity so shapes keep roughly their original weight rather than eroding or fattening. If you need real partial transparency in an animation, GIF is the wrong format.

600, but the number of frames is only half of it - what actually matters is frames multiplied by frame size. Every frame is held in memory at full size until the file has been written, so 600 postage stamps are fine and 600 frames of 1280x720 would be over two gigabytes and would take the tab down. The page works out what your device can manage at the size you are building at, checks it before any work starts, and if it is too much it tells you roughly how many frames that size would allow rather than just refusing.

No. The palette, the LZW compression and the GIF container are all written by code running in this tab, and a content security policy the browser enforces prevents this page from sending anything to another domain. There is no encoder service and no library fetched from anywhere.