Always hang pictures at eye level.
Arrange the frames on the floor first before even picking up a hammer and a nail. That way you can figure out the exact look that you want in advance. Then take one picture at a time to hang it on the wall.
If you want the look of a larger picture but don't have one, one option is grouping 4 small same size frames together in a square to give the illusion of a larger picture.
You don't always have to use the same size frames either. Different shapes and sizes can look really nice together. Even adding a mirror or something completely different like this cardboard deer can look good.
There doesn't always have to be four in a group. I really like grouping three frames together. One large picture and then two medium size frames, like this.
Don't put one little picture on the wall. It doesn't look good. If you don't have similar pictures, that is alright. Mismatched pictures can look really good together.
Two of my favorite places to buy picture frames are Dollar Tree and Goodwill. It doesn't matter if it isn't the color you are looking for. You can spray paint it whatever color you desire. I don't remember the last time I spent over $1 on a frame.