Hi all,
whether to replace the icon of the cart by a nice png image here is the method that I have used and works.

Edit the icon of the cart module and choose opencart icon (important!).

eg : http://nimb.ws/VXpecJ

Find a png image for the cart module, there is a lot on google, and resize, mine is 50×65 pixels.

In the custom css section add this code by modifying the link to the image with your own link.

 
.fa-opencart:before {
    content: url("http://www.your-site.com/image/data/your-image.png");
}

I have intentionally disabled on cart module:

Show total and Show label, it looks better.

Here is the result :

http://nimb.ws/QEdFpi

have nice day