The Extra Theme┬áhas inherited the Divi Theme’s behavior of displaying a “+” icon when images are hovered over (as well as making them a touch darker). If you’d like to get rid of the plus sign icon, you can do so with the following CSS:

/* Remove "+" from image overlays */
.et_pb_extra_overlay:before { content: ''; }

You can add this to the theme by pasting it into the “Custom CSS” box found in the ePanel at “Extra > Theme Options > General Settings” (at the very bottom).