When creating your own pattern, you sometimes have a picture or design that needs to be converted into a knitting chart in order to be useful as a final pattern. The inherent problem with creating knitting patterns from pictures or other printed material, is that knit stitches are not usually square, and hence cannot use straight grid/graph paper to create the charts accurately. For most common weights of yarn, the gauge of the knit stitches is wider than they are tall. This leads to the need for a landscape-oriented rectangular graph paper to do charting on.
If you need blank graph paper to chart on, Sweaterscapes has a collection of pre-generated graph papers in various proportions to download here. If you are working with a ratio that is not pre-generated there, you can create custom grid paper for printing at Incompetech.
If your desired image is already a digital photo, the Knit Pro Web Application can take a single photo and approximate it onto square or 5:7 aspect ratio grids.