After the upload of an image you can put three checks on each one.
Preview image: This picture will be displayed at the tour search – so basically before clicking on the tour and opening it. If you don’t choose a picture as “preview image”, the image that was uploaded first, will automatically be used as preview image.
Gallery: All pictures that are marked with a check are displayed in your gallery. When you publish the tour, all these picture become visible for other users who open your tour.
Topo: Topo is mainly used with maps (topographic maps). They are then displayed in a different format (pictures with topo addition are larger on PDFs than pictures without the topo addition, which is especially important for the readability of maps).