If you want to import an image as an image plane:

1) in one of the orthographic views (top, front or side), choose View -> Image Plane -> Import Image...

2) Browse to your image and choose it.

If you want to import an image as a texture or material:

1) Create a simple shader; blin, lambert, phong, etc...

2) With the shader selected, open up the attribute editor (ctrl + a); across from "color" there is an image of a black-and-white checker. Click it.

3) A window should open up; choose file.

4) Your attribute editor should update to show the file node of the material network; next to the blank line, there is a folder icon. Click it.

5) Browse to your image and choose it.

Hope that helps ^_^