Using FTP is really easy, and if you're managing a zenshop (or any other webshop software) then you'll need to know how FTP works.
You could install ImageHandler 2 (IH2) and this would provide an interface to upload additional images.
There IS a way in admin, but it is time-consuming...
When you ADD or EDIT a product, there is an opportunity to UPLOAD an image when you fill in all the product details.
Let's say you have a product - "Candy Bar"
It has THREE images:-
candybar.jpg ..........(The main image)
candybar_01.jpg .....(The first additional image - NB how it is named)
candybar_02.jpg .....(The second additiona image - also named properly for zencart)
ADD the product using the add-product edit feature in admin, and with it, upload the FIRST image - candybar.jpg
Make sure the product is added to the dbase, then, come back to edit it a second time.
Leave all data that's already there intact.
Scroll down to image upload
UPLOAD the second image: candybar_01.jpg
This will OVERWRITE the REFERENCE to the original image, but does not remove the original image from the server.
Complete the product edit process.
COME BACK to re-edit, then upload the THIRD image in the same way.
Finally, come back to edit for the last time, where (in the field titled: Or, select an existing image file from server, filename:) type in candybar.jpg...
Thus restoring the ORIGINAL candybar.jpg as the primary image.
Now, you will ALSO see all the thumbnails of the supplementary images.