I am using Startup Template trying to edit it and I have found I cannot replace the image in the overlay in the middle of the page. I see the three images but when I double-click on any of them the only option is inner text settings. I do not see any background image associated with the Intro Block Overlay.
Welcome to the forum - hope I can help with this one!
These images are actually set as background images on the ‘Home Intro Link Block’ elements rather than the Intro Block Overlay, you can select these elements in the Navigator on the left and then change the background image here: