Hello, in all other themes I can but not in Zen page. The Default image display size so that I can define the height, width, shortest side, or the longest side will be equal to image size is greyed out. I changed in MySql Database, I can see the new Size, but the script doesnt take it. I need to display a 250px picture, but it displays the 580 whatever I do.

The Zenpage theme has fixed image sizes built in for layout reasons. Therefore the option is greyed out as they won't work anyway.

Thank you. Is there a way around it please?

Yes, create your own customized version of the theme.