Alt text on Images
We recognised this is a known issue with Weebly that you cannot have a straightforward way of adding alt text for images in E-commerce sites so If going to Advanced > Alt Text in the image editor doesn't work, you can try workarounds for this, by creating separate standard pages that feature a certain product Category. Then you can upload product pictures, then add the Alt Text on the images and links to the products (or Categories if need be).
or go to the product page and add another picture using the build function. It does put this picture under the cart but you can add alt tags to it if you do it that way. Basically you will have the normal pictures at the top of the page and then another picture at the bottom of the page. This is an issue we have raised with Weebly and they are yet to act on this.
You can vote for this features to be implemented by Weebly: