Visit the menu "Window / Arrange / ..." where you'll find a "Float in Windows..." option. If you happened to have several images open simply select "Float All in Windows." From here you can drag between windows. However, to make things easier (depending on how many images are open) I've found the following useful.
1. Select "Float All in Window" from the "Window / Arrange / ..." menu
2. Select "Tile" from the same menu.
3. Select a single images and zoom-out until the images is comparable to a thumbnail.
3. Select "Match All" from the same "Window / Arrange / ..." menu
4. Select an images or layer, hold down CMD + SHIFT, and drag it to the desired canvas.
Note: Holding down "Shift" ensures it aligns center on the destination canvas. Or dragging onto the workspace, as opposed to the actual canvas, can do the same.
...hope that helps.