Hi freelancer,

I'm sorry, but not, you can use only Font Awesome icons for this purpose. 

It is possible, but a little bit of a pain. You need to use a little custom CSS and then change the forum icon setting.

1. For every custom icon, you should have two versions, one for read and one for unread.  Then insert custom CSS in Forums>Settings>Styles:

.wpf-unread-forum .icon1 {
    color: transparent!important;
    width:32px;
    height:32px;
    background:url(  https://example.com/icons/icon1_unread.png  )
}

.icon1 {
    color: transparent!important;
    width:32px;
    height:32px;
    background:url(  https://example.com/icons/icon1_read.png  )
}

Of course using your own image URL and dimensions.

2. Then in Forums>Forums edit each forum, and in the Forum Icon text box, simply insert 'icon1' in place of the font-awesome one.  At least the current wpForo code will insert the 'icon1' class in the <i> element.

You will need to do this for each and every custom icon you wish to use.