I am going insane here!

I am using a template with a Gallery Page, whereby there is 3 rows of 6 thumbnails.
When I try to drag and drop the images i want to use, some of the photos are going into 2 of the thumbnail boxes, I am doing what you said and holding down the Ctrl key when dragging and dropping the images.
I am just not getting it.
I cannot delete or add a row to this area of thumbnails, it seems they are combined together.
PLEASE ADVISE? What can I do?
THANK YOU
LYN