After you add a product if you want to add more images, first you will need to upload those.
On the left hand side of the Admin Click Upload Images.

Once you have uploaded all the images you want to add on, click View Products on the Left Hand Side.

This screen will come up.

Click Manage Images – A popup Window will come up, scroll down and click ADD next to each image you want added onto that product.

If you want a product to appear in more than on Category, you will click Manage Categories, a popup screen comes up, click ADD next to each Category you want this product to appear in.