Hello Alice,
the default color is blue, therefore when you click on an image in 1Ds it is automatically marked as blue and will be displayed in a 2D view marked as blue. When your 2D view has no assigned color, clicking on an image in 1Ds will not lead to displaying this image in unmarked view. You can drag and drop it to this view and by clicking on left and right arrow keys, you can run through all images.