Ok, here is what you need to do. Don't upload second images the traditional way as explained on the Zen Carts site. The Image Handler 2 that I have install on your sites, takes care of things in a diffrent way.
First rule, You must upload the original product image to a sub directory, it cannot be uploaded to the Main Directory. If you have used the Main Directory you will need to re-upload the main image to the new directory you have created. You can do this by editing your product.
Second. Go to in Admin ->Tools->Image Handler 2 and you will get a page with a list of what you have for product. Find and select the product you want to add an additional image to.
When you click display, the product you selected will come up. It will, then display the current image you have for that product.
Find and click "New file"
Browse to the image on your computer and select it. You only need to worry about the field that says "Default Image".
When you find the image you want, click SAVE. I will add the new image to your product. When your done, you should have 2 lines with images in them. You can add as many images as you like.
If you did everything correctly, the new image will show as a thumbnail at the bottom of your product page.
I will edit this page in the very near future.