There is a bug in eZ kernel for the Image attribute concerning the field "Alternative image text". The code only checks if the input field for uploading a image has content when saving the image text. If you are not uploading a new image, but has an existing image uploaded, it will not be possible to add image text.

The bug is happening in this file: kernel/classes/datatypes/ezimage/ezimagetype.php on line 301

I made this change to the code to make it work:

if ( ($contentObjectAttribute->hasContent() || $result) && $hasImageAltText )
{
    $content->setAttribute( 'alternative_text', $imageAltText );
}