The stitches used to create a picture have a pretty strong influence on how the finished product will look. There's a different stitch for every way that yarn or thread can be laid across a fabric, and each one has its own merits and caveats that need to be considered when planning a project. Most of my designs use some combination of one to three of the most basic stitches because of their versatility and durability. Using the fundamentals, pretty much any degree of complexity or subtlety can be achieved. Any decent pattern-book on the market usually includes at least a brief how-to section on stitching, and there are several large stitch reference books available that each do a pretty exhaustive inventory. Check out my References section for some pointers to those sorts of books.
Tent Stitch
(incl. Half-Cross, Continental, and Basketweave methods)
Backstitch
Scotch Stitch