Can I decide manually what the featured image is?

Yes. We support the Facebook Open Graph og:image meta tag, which you can use to specify the featured image in your site’s code. This is the same tag that determines the featured image displayed when someone shares that page to Facebook, and is widely used by websites. The og:image tag needs to be specified it in the HEAD section of your web page.
This feature is available for all paid plans, but it is switched off by default; to switch it on, simply go to Settings in the AddSearch dashboard, and under Image Tag click on “On”. The minimum size of the image is 140 x 140 pixels and the image shape should be relatively square. One side of the image can’t be more than three times longer than the other.