They are actually both not getting "big" at the same time. When you hover over one of the images, it swaps, thus changing the size of the paragraph "box." Since both images are in the same <p> tag, the other image's position is being adjusted as well.
I don't have much experience with image swapping, but I have a few suggestions. First try removing the <p> tag from around the images. See if that at least temporarily fixes it. You could also try doing an align='top' or something similar in a stylesheet (preferably a stylesheet since align is very much deprecated).
Lastly, again I am no expert, but should the onmouseover and onmouseout attributes be in the < img > tag itself and not the < a > tag? Try moving them into the < img > tag.
EDIT: Added spaces to some tags to prevent phpBB parsing them as real HTML.
The primary purpose of the DATA statement is to give names to constants; instead of referring to pi as 3.141592653589793 at every appearance, the variable PI can be given that value with a DATA statement and used instead of the longer form of the constant. This also simplifies modifying the program, should the value of pi change.
-- FORTRAN manual for Xerox Computers