Set New Group Permission - Everybody

Go to the Edit Permissions page and set a new Permission Group called Everybody and set the Permission to View all versions - That way people can see the large photos you have loaded.

Add new Permission Group (Everybody) to the Group Permissions list

Once you have created an Permission group called Everybody click on the Add Perrmissions box to add it to the list.

Then add your items

Finally , go to the Add Items menu and start loading your photos. Make sure the box at the bottom of the page is checked so that it adds thumbnails automatically.