If you use HTMLArea to configure html formatting for your product descriptions, you can (as with ANY HTML page) embed links to images - and indeed other types of content.

To make images "visible", the browser needs to know "where they are" on the internet.

For the most part, the images you use will be on your server - in a named directory - and there will be a PATH to that directory.

Heres a simple example:
<img src="http://www.mywebsite.co.uk/shop/images/image_1.jpg">

All you have to do is:-

1. FTP your images to an appropriate directory on a server linked to the internet.
2. Specify the PATH to that image when you want it to show up in your product description text.

NB: The PATH could be absolute - in other words, the FULL http://www.mywebsite.co.uk/shop/images/image_1.jpg

Bear in mind that this may invoke the SSL warning that the page contains both secure and insecure elements because the path to the image.

Zencart cobbles all pages from various templates and data sources, but also puts into the <HEAD> section of every page, the BASE HREF, which is equal to the root directory of your webshop.

As this BASE HREF governs all internal hyperlinks, you could simply FTP images to your IMAGES folder, and create links such as:
<img src="images/image1.jpg">
and the BASE HREF will ensure that the path is fulfilled.

HOWEVER...

I have noticed sometimes that if we use the <img src="images/images1.jpg"> format, then later recall the product to re-edit the text at a later stage, it will strip in the word "admin/" infront of "images/" for some reason:-

So, where we started with
<img src="images/image1.jpg>

We sometimes find
<img src="admin/images/image1.jpg>
being thrown back at us when we recall the text for later editing.

This may be due to the fact that HTMLArea works within the ADMIN directory of the store, and it assumes that if you are talking about the images folder, then you are talking about the images folder that sits in ADMIN.

Two ways to get round this...

1. Put a COPY of your required images into ADMIN/IMAGES (in addition to the IMAGES folder in the shop's root directory;
2. Edit out the ADMIN by toggling to the HTML code in HTMLArea, returning the link to <img src="images/image1.jpg">

I tend to use option 1 above - then I know that if I miss a <img src> link that still points to "admin", the image will still show, as there's a copy of the required image there too.