You might want to try using just "onClick" instead of mousedown.  see if 
that helps. 
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Is it possible to run javascript on a onmousedown over an image,
or invoke a function
what do i need to change to make the following code work

<INPUT TYPE="image" SRC="larw.gif" 
onmousedown="javascript:this.form.Field2.value++;">

im trying to increment a value in a text box(field2)

thanks for any help