If you are really going simple here and have a single directory for all images, have the THUMB images created in a thumbs directory inside your images directory.
You can even append them with a th_ on the front so that they are slightly named differently to avoid any confusion coding.
This way - you have the PATH to the images directory (the full size)...and from that path, you always know that the path to the thumbs directory is one more folder deep.
