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Custom Image Attributes is a lightweight WordPress plugin which allows you to add custom attributes to images on your website. You can add attributes to single images or all images.
You can use this plugin to add attributes to images as required by some other plugins, such as for lightbox, analytics and seo purposes. Add any attribute you like. Examples include;
data-lightbox=image-ad1 
title=demo 
alt=demo 
fetchpriority=high 
click-id=promo1 
or id, style, class, data, toggle & more
Free Features 
Add attributes to all images on your WordPress website. 
Add attributes to single images 
Remove attributes 
Disable native lazy loading for an individual image
Get Support 
Get limited support for this free plugin via the support tab.

Custom Image Attributes

Add custom image attributes to any image. Great for adding attributes to images for lightboxes, SEO, analytics & more.

Restores WordPress default functionality prior to version 4.7 where image ALT attributes were automatically generated and put into the HTML code. The ALT attributes were generated from the Caption (if it exists) else from the Title (if it exists) of the image in the Media Library. This plugin utilises Titles in generating ALT text, but does not utilise Captions. The Title used by this plugin, is the main Title of the image as found in the Media Library, not the HTML TITLE attribute of the image (that is something else, which this plugin ignores).

Automatic image ALT attributes

This feature is a time saver for me as my site is image oriented.

Thanks

Automatically generates ALT attributes in image HTML. Restoring WordPress < 4.7 functionality.

Custom Image Attributes vs Automatic image ALT attributes