Hello @artistik,
The problem in your case is most likely the fact that you are using '«' instead of double/single quotes for the src attribute.
If the .png is bigger than the icon, you will need to use some CSS in order to resize it as well. You can also use the height/width attributes.
Here is an example of how it should look like:
<img src="https://url.com/themes/classic/modules/ps_shoppingcart/trolley.png" alt="shopping-cart-icon" width="25" height="25">
Make sure that you can also access https://url.com/themes/classic/modules/ps_shoppingcart/trolley.png using the browser, otherwise it will not show up.
Please also make sure that in Admin -> Advanced Parameters -> Performance, in the Smarty tab, the Template compilation is set to either "Recompile templates if the files have been updated" or "Force compilation".