Flamming grenade icon on upper left it 's for indicate : grenadier infantry
it use for grenadier ( no guard) , Guard grenadier and Old guard
Think you are misunderstanding, the picture of the guys, for Old Guard it should look like the two guys on the left, marching with bayonets. Grenadiers look like the two on the right, standing, holding their muskets ( muskets not rifles right? ).
The Old Guard unit stickers had one sticker with the Grenadier look, like you can see in the middle.