We have no way to specify a custom button image URL that would be included in the button codes we generate for you. However, your Add to Cart buttons follow a common format where only the &i= value (Item Number) differs from one product to another -- in this example Add to Cart code, XXXXXX would be the product's Item Number:
Your View Cart button code is unique to you but the same for all your products, so you only need one copy of that code on every page; typically, this would go in a common header/footer/sidebar area that gets inserted into every page.
We can also help you import your initial batch of products all at once, and this would allow you to specify a custom Item Number for each product, so you would have a record of those numbers at your end to facilitate placing Add to Cart buttons in your site. Just email support to request our import template file and further instructions: