No “alt” attribute for pictures is dislayed at the slider in the source

Hello, my problem is, that the "alt" attribute for images at the slider in the clean journal theme ist not displayed in the source. At the media i have set a alt-Description, but it isn't "transportet" into the slider when used there. w3 Validator says: ____ An “img” element must have an “alt” attribute, except under certain conditions. For details, consult guidance on providing text alternatives for images. ____ ... and my customer wants to have it clean for w3c Has anybody a solution for this? Thanks a lot Greetings, Sören from Germany :)