I'll describe quickly the principle behind the database's reference to images, and how this relates to the ACTUAL images themselves.
The database contains the NAMES of the images that are associated with your products - so in the dbase tables, you will see the references to such names:
eg:
product1.jpg
product2.jpg
product3.jpg
...etc
Then, in your images FOLDERS, you house the actual image files, and each image will itself be called an appropriate name:-
product1.jpg
product2.jpg
product3.jpg
Using the php code inherent in zencart, an ASSOCIATION is made between the information in the database, and the images in the images folder(s), when a page is "cobbled together" for display in the browser.
If the data in the database cannot correspond with the actual file name, then it cannot associate the two, and no image will be rendered.
So, in short, the NAME of the image in the image folder, MUST BE identical to it's associated reference in the database.
There are ways to automate the renaming of images, but this cannot be done via the ZC admin panel.
You will have to do this on your local computer. I use a program called Ulead Photo Explorer, that can do a lot of "batch" processes with image files - including re-naming them.
Once your images are re-named, however, you will have to alter their associated reference in the database too! The best way to do this is to use easypopulate to "compile" your product data as a spreadsheet (which you then port to your local drive) - after which you manually edit each image name. When complete, you use Easypopulate to "overwrite" the database, thereby inserting all the new names - essential if the association feature is to work.
I always recommend that images are given identical names to the product codes (the latter should be designed according to accepted SKU principles, and be alphanumeric...)
... so if I have a product where the product code (model number) is:-
9984537
... then its corresponding image will be
9984537.jpg
In this way, I can immediately see if an image is relating to the correct product, as the SKU number is the "bible" reference.
Additionally, if you have additional images for that product, it's then also very easy to invoke ZC's associated image feature:-
9984537_01.jpg
9984537_02.jpg
Hope this helps.
PS: If you are using IH2 (Image Handler) there's no need to have MED and LRG images - just put all full-size product images into the main images folder... IH2 will do the rest automatically!