.image-overlay.overlay-type-extern .image-overlay-inside:before {
content: ‘Your custom text goes here’;
font-family: ‘entypo-fontello’;
}

Thanks for this Yigit!
May I ask, How would I use a font icon? I know I can just type custom text here, but how canI add an icon from the default Entypo or from uploaded ones from fontello?
An example would be nice so I can see the structure of the code.

thanks!