As long as your phone doesn't automatically convert it to MMS then it shouldn't charge as long as you have enough texts left on your tarrif.
I have MMS disabled on my account just to be safe, I think I can still receive them without being charged, but who would actually use SMS for images etc now that there are apps for this that do it for free anyway.
Some emoticons are interpreted by the SMS software from such inputs as
: - )
The three characters are sent in the text and it is decoded as a smiley at the other end if the handset supports it, otherwise it just appears as those characters (for example on an old handset). The display appearance of the icon on one handset might be different to the appearance on another because it's controlled by the receiving handset. For example, my Android phone displays barely legible icons (can't tell the difference between a smile and a frown) but it's perfectly clear on my wife's iPhone - all from the same sequence of three letters in the text.
If you send an emoticon which is surplus to the small set of smileys that can be sent as SMS, the actual data contained in the picture is sent, which is why it needs to be converted to MMS.