Well I got no clue what those emotions are but oh well seems like smilies or something that replace a certain text in some program automatically... (am I right on this?)

now if you want to make a custom file format you can just do it by using get and put in binary mode.
Now I really don't understand your specs since I got no clue what a one byte string is supposed to be... it is described as "text to replace with icon" but one byte could be only one character... so are you sure it is one byte? If it (the one byte string) would be a token to devide different parts what is it and why would there be one in the beginning of the file...?

so now to the 4byte string... is this supposed to be the picture? Are those pictures black and white? That way you could have 8 pixels per byte, 4*8 would make 32 pixels but don't you have 16*16=256???