@MarkWGabbyLi The second format's for "letters" used in puzzle names. A 13-bit number and three 12-bit numbers together store the 49-bits of a 7x7 pixel sprite.
There's also some special formats for one-use things, but those two formats do most of the work.
@MarkWGabbyLi There's actually two compression formats at play - the first is for puzzle images and thumbnails, where a 10x10 pixel sprite is encoded as ten values, each representing a 10-bit column of pixels.