the picture in your "lobby" / personal lounge is your Profile Picture, the same as your picture in the GT Life Menu (upper left corner). The picture can be your avatar or a picture you took using Photomode. To change pictures, just go to the Photos menu in GT Life. Choose one of the photos you have saved, then choose Set As Profile Picture. Or if you want your avatar to appear as your profile pic, go to your profile (highlight the portrait in the upper left corner of the GT Life menu), select it, and select the camera icon (Take Random Picture). The game will generate 9 different poses for your avatar, from there you can select one and have it set as your profile picture.
