Howdy davidemacek,
You can accomplish this with CSS. The following line should work, but since it involves a CSS selector that is not compatible with older IE versions:
.tribe-related-events-thumbnail img[src="http://yourdomain.com/wp-content/plugins/events-calendar-pro/resources/images/tribe-related-events-placeholder.png"]{
display:none;
}
Or this should work, although it might depend on your theme a bit. Most themes add the ‘.wp-post-image’ image class to custom images, but not to default ones like the calendar. If your theme does this as well, this is very cross browser compatible:
.tribe-related-events-thumbnail img{
display:none;
}
.tribe-related-events-thumbnail .wp-post-image{
display:inline-block;
}
Did that work for you? Please let me know if I can help further! Thanks.
– Brook