We used universal way - the emoji inserted as special unicode character, this special character is interpreted the different way on different platforms. The text can be copy/pasted and it will work everywhere on modern devices and platforms. However the look of the particular image can be slightly different - for example on one system it is color icon while it is white/black on another system.

Another way is old school smiles displayed as images - this way it will look the same everywhere, every see more smiles requires separate image and can't be safely copy/pasted.