Its a quite easy, please go to ..\themes\goodstore\templates\header\top_bar\ and open file woo_cart.php. On line 3 you should see this icon-cart3, simply change the icon-cart3 for another one (e.g. here)

We got this

If you want to change image instead of icon, please add this to custom css (appearance->theme options->custom code->custom css)

.topbar-title-icon.icon-cart3 {display: none;}

Then go to

..\goodstore\templates\widgets\ecommerce_widget\

and open file

woo_cart.php

and add this html tag on line 3 (after 2 line). Upload your cart image to wordpress dashboard->media and copy its (image) path and that path paste instead of

<img src="CHANGE_ME_TO_YOUR_OWN_IMAGE_PATH" />