Hope you refer to "What we do section".
If you go to extensions -> modules and edit what we do module - you will find "data source" tab on the right - go there and you will see that it takes as a source all articles from k2 by default.
Now go to corresponding k2 article, edit it and go to "Image" tab just above tineMCE editor - there you can change the image.
Last but most important - NSP module caches the images so the change will not be visible right away. To make it faster, you can either change Thumbnails settings in NSP module, or just go to:
/modules/mod_news_pro_gk5/cache/
folder on your server and remove all files which name fits:
k2.items.cache.*.jpg