Changing icons (unicode) not font awesome

Hello,

I need to change an icon which is activated when you click on a gallery picture.

It appears here :

.icn-expand:before {
content: ‘\e816’;
}

Font awesome and Unicode have not exactly the same icon i need.
my icon name is : myicon.png

Which files should I modify to do so and what should i put instead of the existing code.

Thanks in advance.

Regards