How do I mount large frames pictures without hitting wires?

They make picture hangers that will hold up to 250 pounds so just get the proper sized hangers and you'll be fine. The studs are never where you want them anyway. I wouldn't worry about the wires too much, unless you're putting 3 inch screws in you shouldn't be able to hit a wire in any case, they are supposed to be in the center of the stud which with 1/2 inch drywall means at least 2 inches from the outside of the wall. Anywhere they run through the studs they are supposed to be protected by a metal plate. Supposed to be is the operative phrase here. Just use the right hooks and you'll be fine. For big frames it's sometimes easier to use two hooks instead of one. Hang them level with each other and it will make your pictures much more likely to stay level once they are up. I'd suggest you not hang any large mirrors with wire, the weight can cause the frame to bend which can crack your mirror. The mirror should have two large loops on the back, one for each hook. Oh, make sure the hooks are evenly spaced from the top of the mirror if you hang it this way, otherwise you'll put your hooks level, but the mirror won't be. Feel free to send me a message if you have anymore questions.