This is a basic summary of the information that can be found in this post at TeX Stack Exchange. I've put together a template to make things easier and walk you through the process step by step.
1. First download this template: WriteOnPic (.tex)
2. You'll need a picture, so I've provided one for illustrative purposes: Apicture (PDF)
3. Put the WriteOnPic.tex and Apicture.pdf in the same folder, open up WriteOnPic.tex with your favorite IDE (I've use Gummi in the picture below) and change MyPicture.pdf to Apicture.pdf
Notice that your picture has a grid with numbering on top of it. We can remove the grid and numbering later but for now the coordinates will help us determine where to place our graphics and text.
4. Adjust the grid parameters so that it covers your picture. You'll then have to adjust the numbers in the \foreach statements so to go along with the new grid.
5. Insert the text and graphics using tikz commands. Refer to "Minimal introduction to tikz" if needed.
6. Once you've got everything where you want, comment out the grid and numbers from your $latex LaTeX$ code [put a percent sign before the 3 lines under the \includegrachics command as shown in the picture below]