Assuming you have your own button image and know the path/url to it, you would add the following in your page's HTML...
<a href="index.php?l=addtocart&product[id]=99&product[quantity]=1"><img src="images/buynow.gif" border="0" /></a>
Edit: Keep in mind that the path in this example is relative to where the shop files are. So if you are creating a link to the shop from the main/public folder on your site, and your SunShop installation is in a folder called "/sunshop", then the link would look like...
<a href="sunshop/index.php?l=addtocart&product[id]=99&product[quantity]=1"><img src="images/buynow.gif" border="0" /></a>
Obviously, you'd replace "99" with whatever your product's ID actually is. We suggest using relative paths, and you need to make sure the path you're using is correct. The above example assumes the image is in SunShop's main "/images" folder....though it can be anywhere you like.