Table 4 - If you use the images as links, you'll want to make sure that the attribute BORDER="0" is in the image tag or you might get not only spaces, but blue boxes around the sections (blue or whatever your link color is). Actually, it is good practice to include BORDER="0" in there anyway, because some browsers may default at 1.
Keep in mind that these things can be happening one at a time, or in combination. Also, even with these three direct causes eliminated, you may find that something else is causing the problem indirectly.