I omitted the texture coords from my first picture. Here is another picture with the texture coords.
They are specified in (u,v) format where u is the horizontal and v is the vertical.
They are specified in range 0.0f to 1.0f - which means that your texture coords are independent of texture size. If you specify .50f,.50f on a 100x100 texture it will be 50,50. If you specify .50f,.50f on a 1000x1000 texture it will be 500x500. Notice both correspond to the center of the texture regardless of size. These are called normalized texture coordinates since they always fall in the range 0.0f to 1.0f.