3.5.  Drag and Drop

Alternatively, once you have found the file, you can click on its icon and drag it into the GIMP Toolbox. (If instead you drag it into an existing GIMP image, it will be added to that image as a new layer or set of layers).

For many applications, you can click on a displayed image (a full image, not just a thumbnail) and drag it into the GIMP toolbox.