Does it require the entire buffer?

[gajus]

Is it enough to read just a chunk of the image? Loading 20MB image into memory is suboptimal.

[qzb]

In theory you can provide only part of the image, but it is hard to estimate how much data is needed to determine if it's animated or not. It depends on type of image and it's content.

It's possible to implement support for streams, but it's not a trivial task, since it would require a new API, and significant changes in current implementation.