You submit an image to the group by clicking on the "My Group Images" tab then click on the "Non-Group Images" button and you'll see a list of all your images, then click on "add to group" at the far right of the image you want to submit. Then you'll have to wait for the group moderator to approve your image. Keep in mind some groups have essentially abandoned. I give them a week to approve my image, if they don't I assume the group is no longer active and leave the group.