The third byte might just be a format version number. It's usually good practice to include such a thing in a binary format.

As for the bytes in grey, think about what information you are missing: if this file, BLATT.RES, holds animation frames that are supposed to be played together but which are of different sizes, then there is probably a relative position for each frame of the animation. There might also be some animation speed information somewhere, perhaps a per-frame display duration.

I'd try to see if the bytes in grey may contain those things.