Do you use a specific plugin for the Enfold demo to handle multiple image sizes ?
If we look at the blog grid page : http://kriesi.at/themes/enfold/blog/blog-grid/
All the images are 495×400:
Then if you look at the post page: http://kriesi.at/themes/enfold/2014/04/28/cryptocurrencies-are-booming/, all images are 710×270.
How do you do that ?
That is WordPress resizing. If your image dimensions are quite a bit different from the image size you select then WordPress may create a new image and crop it a bit. For example if your using a large image and select the thumbnail size ( 80 x 80 ) with our blog shortcode then WordPress will probably crop the image a bit to 150 x 150 so it displays better.