Practically any graphic editor can be used to design/construct smilies. the gimp is a nice freeware graphic software that is more than adequate to allow you to design smilies. Alternatively, if you have money to spend then photoshop, paintshop pro are very good programs to use. I'm not exactly sure but I think there is a 30 day trial of photoshop that you can download and use if it's only smilies you want to design.
You can download the gimp for free
Here or go for photoshop
here with the restriction of a 30 day trial.