Great News!
The Add-ons (that you can generally trust) are in the DOWNLOADS section of this forum (see link at top of screen).
However, these do not come with any guarantees. You install and use them at your own risk!
Remember too, that some add-ons may "conflict" with others, so be cautious and judicious in your use of modules.
Basically, the add-ons are all zipped files, which you download to your local drive. You then "unzip them" and navigate to the root folder - which in most cases will show either an "admin" folder or an "includes" folder (depending on whether the module is for the admin part of the site, or the front-end (catalog) part.
All reputable modules will have a readme.txt or install.txt file, with instructions of what to do.
As many modules contain files that may OVER-WRITE core files, it is important that you observe the principle of the OVER-RIDES SYSTEM.
Good add-on modules will have factored over-rides into their structure, and, in general, the author will have placed files that supercede core files in a folder called YOUR_TEMPLATE.
So, you have to go through the directory tree of the add-on module, looking for instances where folders have been called "YOUR_TEMPLATE".
You must then RE-NAME such folders to the name of the template you are using.
In a standard ZC install, this the "classic" template. Many people use other templates, but whatever your template is called, you MUST name all "YOUR_TEMPLATE" folders to the name of the template you are using. And this MUST be done before your FTP the files to your server.
Some modules may require "tweaks" to the database, in which case there will be files with a ".sql" extension. These are NOT ftp'd, but the SQL code they contain has to be "patched" into your database. So you simply copy the SQL code to your "clipboard" and "paste" it into the SQL Patch facility under TOOLS in your ZC admin panel.
When you've made all the necessary alterations to folder names, and have READ THE INSTRUCTIONS for the add-on, FTP the folders/files to your server.
Now, the module you are interested in is probably IMAGE HANDLER 2 (IH2). This is a great add-on, and not only facilitates zoom features, but does a lot of image management besides.
Go to the downloads section and collect the zip file for Image Handler for ZC 1.3x, and follow what I say above.
When you've ftp'd the folders/files, you should see IH manager appear under your TOOLS menu. You need to then install and configure it - just follow the onscreen instructions.