In the CMS you can use the various editors to add alt-text to your images added to your banners and content pages.
The system does not allow for alt-text to be added to system generated pages e.g. main product images or thumbnails, blog images. You can add alt-text to any custom images added to the product content areas or blog content.
Images applied as backgrounds e.g. to contentbox sections are considered decorative rather than content by browsers and cannot have alt-text as an option.
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Adding Image Alt-text via the Drag Drop editor