No, havent found the problem. The mouseovers are showing fine because I've changed the link to one button since typing the question. It still doesn't work when I add a second button.
I've since added the second button again, so you can see that it is no longer working.
Very strange.
http://thruwayspring.coffeecup.com/Heavy_service.html

okay, figured it out. Forgot to change the second button set of "myimage.src" to "myimage2.src" as shown in the example here http://progrower.coffeecup.com/
PNG mouseovers are working now. Hope this makes sense.

This is what the code looked like in the end

<a href="http://thruwayspring.coffeecup.com/Tires.html"
onMouseover="myimage.src='images/button_tires_mo.png'"
onMouseout="myimage.src='images/button_tires.png'">
<img src="images/button_tires.png"
width=200
height=125
border=0
name="myimage">
</a>

<a href="http://thruwayspring.coffeecup.com/H_exhaust.html"
onMouseover="myimage2.src='images/button_hexhaust_mo.png'"
onMouseout="myimage2.src='images/button_hexhaust.png'">
<img src="images/button_hexhaust.png"
width=200
height=125
border=0
name="myimage2">
</a>

Glad it's working for you. BTW your site is looking pretty good!

Thank you! So much more work to do. Now I have to figure out how to adjust the spacing of the buttons. I don't suppose you know how to do that, do you?

Nevermind, figured it out

hspace="10" vspace="10"

hspace being horizontal spacing and vspace vertical