Virtually every PowToon Studio style has speech and thought bubble options designed for that specific style; you will find them in the props area. You can also import your own images. A great source of custom speech and thought bubbles, along with loads of other goodies is http://www.elearningart.com/. You can also find them at http://www.elearningart.com/Speech_Bubbles_s/60.htm.
If you are using these, here are a few general rules that I use to try and add a little professionalism. You can (of course) disagree with them; however, once again, just consider whether they add something to your project: