Read this point: http://www.minibb.com/forums/7_843_0.html#msg25842
This problem is related only to your own project and/or mistake. We're investigating such kind of tasks only for a fee (in that case, I am sure the mistake is on your side and there is nothing to fix in the smileys addon itself).
Purchasing premium addons allows you to contact us privately in the case if you have problems with the purchased software (in the LICENSE.txt it says precisely what kind and within what time), but it doesn't affect the common version of miniBB or other addons.
What I could say about your problem without getting deeply into it - what you have on message form, is just a straight HTML. I don't know what you have put in main_post_form.html template (can't determine it w/o seeing the source) but it looks you just put the smiley codes not the smiley images themselves as you should do. Smiley codes work when you post a message, and they are transformed into images then; but for the static form, you should have static HTML, that's the simple idea...