Can I Get Pictures Made Into Posters?

Is there any website/store where I can send them a picture and they can make it into a poster? Also, does it have to be my own picture, or can it be a picture of the internet?

Google can be your best friend. It has to be your own. And it will depend on the photo. Some photos will have a lot of grain / noise if you try to enlarge them.

Here in the UK you can walk into many Max Speilman stores and have images that you have the rights to printed "while you wait". The quality is okay, but not really good enough to sell, for that most large towns and cities will have a commercial printshop. Oddly the printshop cost is usually lower, the quality higher, and they can offer a wider range of print medium choices (though it may take time to get in custom stock).

Nobody will print up a poster of someone else's work for you, that's copyright infringement. It would have to be a picture you did.