Firstly, it I suggest that resizing you image before hand is always a good idea. I would highly recommend - Image Resizer Clone
Secondly, make sure where possible that your image names are always one word and do not what special characters in (such as " ( " and " ) "). This was a tricky one to notice.
Now when you wish to add an addition image. Click on the Product Item Name, then on the left hand side click Add File/Image
Choose you file and hit Apply in the top right of Joomla.
You can remove the tick if you wish to not resize the file your are uploading.
NOTE: if you want to add and additional image, be sure to follow this process from the beginning again - exactly! If you do not, you will be overwriting this existing file (placeholder) that you have just uploaded.
A quick preview or F5 of your front end should show show you a new thumbnail. If the images does not load up from there check how you have named your file.