Wordpress Theme Modification

General information for the website: Craft Website
Kind of development: Customization of existing website
Description of every page/module: What I am looking for is the option to be able to add more images to the existing "featured image" on the portfolio section of this site. I would need to be able to add up to 20 images and for them to be viewed similar to the one on the home page. Any questions just ask please.

http://www.ardswebdesign.co.uk/ceilidhcraft/?portfolio=brooch-bouquets

Thanks
Description of requirements/features: What I am looking for is the option to be able to add more images to the existing "featured image" on the portfolio section of this site. I would need to be able to add up to 20 images and for them to be viewed similar to the one on the home page. Any questions just ask please.

http://www.ardswebdesign.co.uk/ceilidhcraft/?portfolio=brooch-bouquets

Thanks
Extra notes: