Hi,
this photo with group of people ?
BY default JomSocial loads that image: components/com_community/assets/frontpage-image-default.jpg
but in that template we override it with
.joms-landing__image {
background: transparent url("../images/guest-bg.jpg") no-repeat scroll 0 0 !important; }
(CSS) templates/gk_msocial/css/jomsocial.css - line 623
(IMG) templates/gk_msocial/images/guest-bg.jpg