Browse to the category that contains the product for which you need to add images by going to Catalog>Categories/Products:

After clicking the pink circle with the ‘i’,  you will see this screen:

Click the “new file” button and you will see this screen:

Click the Browse button next to the Default Image File box, browse to your file and upload it.

That should do it…if you have problems with this, check your file size.  Files that are too big can cause a problem.