Adding ALT Text to an image will depend on your website's CMS. This may differ depending on the plug-ins you have installed and how your site is set up, so if in doubt, reach out to your CMS provider.
Wordpress
Log into the back-end of your website.
Navigate to your media library.
Identify the image you'd like to add ALT text to. Click on this image to edit it's attributes.
Fill in the ALT text on with a description of the image.
Save the image.
Wix
Click the gallery in your Editor.
Click Manage Media or Change Images.
Click an image to select it.
Enter a title for the image on the right.
(For Wix Pro Gallery) Edit the alt text for your image under Alt Text if you want it to be different from your image's title.
Shopify
From your Shopify admin, go to Products.
Click the name of the product that you want to edit.
From the product details page, click a product media item to see the Preview media page.
Click Add alt text.
Enter your alt text, and then click Save alt text.
Click the X to exit the preview page.