Ugoira is a custom animation format used by Pixiv. A Ugoira is a Zip file of images, played back using a custom Javascript player.
To download a Ugoira, use the "Save as video (right click and save)" link beneath the image. This is a WebM conversion that can be played on regular video players, but is not lossless in quality.
To upload a Ugoira, paste the Pixiv URL into the source field on the upload page, or upload using the bookmarklet. Uploading a Ugoira file from your computer will not work.
Technically, a Ugoira consists of Zip file containing each frame in JPEG or PNG format, and a separate JSON file containing the delay between each frame. Since each frame has a separate delay, it's possible for a Ugoira to have a variable frame rate. A custom Javascript player is used to play animations back. Since the JSON file containing the frame delays is separate from the Zip file containing the images, saving raw Ugoiras and playing them outside of a browser is difficult.
Ugoira is a custom format exclusive to Pixiv. Animations from other sites cannot be Ugoira.
