Mouth practice

The jaw isn't something I've studied very closely and it shows. Here are some of my study images, all drawn in ballpoint and referenced from stock photos, etc. 

Publishing anything directly derived from images not your own is *technically* illegal, but it's technically illegal in the same way publishing fanart is technically illegal--nobody tends to go after you until you're selling it. Nevertheless, there's so much misconception about copyright flying around I thought I'd make mention of it. If the owner of the original images were to find this and ask me to take it down I would be legally obliged to do so. 'Giving credit' isn't a legal out; it's just courtesy that a lot of artists ask for. Legally, a copyright holder of a character, say, from My Little Pony, can send a C&D to a fanartist to tell them to stop publishing fanart online or anywhere, and they are within their legal right. A person does not have a right to publish fanart or derivative art without permission. In general, though, the copyright holder of a photograph doesn't have a problem with someone sketching from that photograph for learning purposes (which is legal, anyway, so long as you don't publish or sell it) and then showing it off (illegal but few care).

This is an important distinction when moving from merely practicing to doing work for hire. It's easy to think: Oh I'll just copy this directly and nobody will know! but dooooon't do it. Be aware of what is merely a bending or overlooking of the rules. 

P.S. these were all done in ballpoint. The blue on the one was added after the fact for the purpose of placing the jaw. Drawing in pen or some other non-erasable medium helps to tighten up form and forces someone to consider angles and shapes more thoroughly than pencil or digital. It's harder, but as is so often the case, going the hard route yields more fruitful learning.