this is the answer of Clearfy. This solved the problem:
wpsp_image_title filter removes the title attribute only in the link , but not in the image.
In Clearfy there is a parameter SEO -> Automatically set the alt attribute. Turn it off.
By the way, I checked with Gtmetrix and got a warning about the image sizes. I’ve seen in the forum that it is a known problem. Have you an time estimate for the new resizer?
Thank you for your help.