Hi
You can also simply grab the image you are inserting and drag it down to the timeline.

Drag it left to right along the track .

.....and at the designated join between clips you should see a white arrow appear......

Release the image when this appears and it should insert between the clips.....

If you drop it ON a clip it will replace that clip.:o (But you can click the Undo arrow at the top left to try again.B)!
Nat