problems with vbcode.js

i did a little mod to the button code. instead if using:

<input type="button" class="bginput" value=" B " onclick="vbcode(this.form,'B','')" onmouseover="stat('b')" title="BOLD (alt+b)" accesskey="b">

i used:

<input type="image" border="0" src="images/vbcode_b.gif" width="15" height="15" onclick="vbcode(this.form,'B','')" onmouseover="stat('b')" alt="BOLD (alt+b)" accesskey="b">

that results in a wierd code event. it's forcing me to submit automatically the post without clicking the button submit. any idea why? and also what should i change in the java script?