"Drag and drop" involves clicking and holding the left mouse button down on the file you wish to copy or move, and then "dragging" it by moving the mouse with the button still held down until the file name is in the new location. You can then "drop" it by releasing the mouse button. Drag and drop works equally well in Linux and Windows. The photo site also offers an alternative method by just clicking on the "Select" button and choosing the file in the file manager window that opens on your PC.