Thanks for taking the time to reply.
The sprites definitely have images; when the game is run the correct images are shown.
To add a sprite, I am double clicking on the layer that I am using for objects. I then select "Sprite", then click the cross-hair to add the sprite. I then double-click on the empty sprite to show the "Edit image" dialog. I then click on the folder icon and select an image for the sprite. The correct image is then shown in the "edit image" dialog. I then close the "Edit image" dialog by clicking the X in the top right-hand corner. At this point, the sprite is only displayed as a wireframe, but when I run the game, the sprite is shown correctly in the game.
Regarding layers, I have created a separate layer for the background. The sprites are on their their own layer (called "Objects"), and this is the layer that is selected when I am adding/editing sprites.
I have found that if I turn off the background (bottom) layer, then I can see the sprites as white rectangles
I have followed the basic tutorial click-by-click, but I still see the wireframes.