Open the group where you want move the contributions from. Go to page 'Members'.

On your own avatar, click the square blue four box icon:


Then open the first image, and click the 'Add or remove from a group' icon on the 'ACTIONS' menu. Then add the image to another group.



Go to next photo and do the same if need to. But you can only add as many images as the group allows within the limits of allowed quota! You have to either ask admins to increase the limit, or wait until you can add more to another group again. When gone through all the images, you can finally leave the first group, and remove your contributions when leaving.