You are using percentages in several styles which aren't working well. You have div.main height: 50%; div.intro height: 100% and class img.moto_intro top: 51%.
If you change
img.moto_intro{
position:absolute;
/*top:51%;*/bottom: 0;
left:50%; /*was 35%;*/ margin-left: -165px;
}
It makes the image stick to the bottom of div.intro. This works in IE6, IE7, Firefox, Opera and Safari for Windows but I often think percentages can cause problems in some browsers.
Changing left: 35% to left: 50%; margin-left: -165px; keeps the bike central in the width of a smaller window better than 35% because 35% is to the left side of the image which means that in a small window of say 500px the image starts at 175px and has a width of 310px so that the far side is at 485px so obviously it is not central.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.