These are amazing you definately have talent! usually the price of a painting goes on $1,00 per sqaure inch, my paintings are uaually 24x24 and these bring in around $400
It also depends on the complexity of the painting.
Also there's art on Ebay and I think on Etsy, since Etsy is for handmade stuff, and there are probably websites that are just for selling art pieces. (Can you tell I'm still in the process of investigating all this? )
Holy crap, you're only 16??!! Your work is amazing!! I actually have quite a bit of luck selling things on my local craigslist. I sell different crafts, but my point is that you can try your hand locally for free- it certainly doesn't hurt to see what the response is!
The horse is great. If you want to your work you should find a gallery that will represent you. Get a show. Even if you get representation at a coffee shop or diner people will be seeing it every day. Start a website or a blog. Get people to look at your work. Eventually they will sell if you get them seen by the right people. If you have money pay an art consultant. A consultant will know the right people and may be able to connect you with people who sell the kind of art that you create.