WordPress makes the management of images easy with the media library.
As described in the article Why add ALT Text to an Image? it’s important to have alt text assigned to an image, for SEO.
In WordPress, to add the alt text click on the media library link, on the left menu.
Find and click on the image which you wish to modify.
As shown in the image above, on the left is a picture of the image and to the right options to be completed.
The entry of interest is labeled as Alt Text. Click on the textbox and enter your descriptive text.
In the above image I gave the image the Alt Text description of Red BT Telephone Box.
The alt text should be informative, but keep it succinct.
I’ve left the Title text as per the original file name of the image (as auto-filled). You may wish to make this more descriptive too.
As changes are made the details are updated. Confirmation of this is a small spinner which appears at the top.
Once completed, simply close on the cross at the top right corner to close the pop-up.