Problems with Blog Featured Images

Hi,

Since I updated the theme a few weeks ago I have been having problems with the size of my featured image. At first the images were cropped wrongly. My colleague went into the deep code and prevented the theme from cropping featured images, and then in the shortcode added

div.big-preview.single-big{ width: 600px; }
.big-preview.single-big{ text-align: center; }
.big-preview.single-big a{ display: inline-block; }

The problem is that it doesn’t display at all well on smartphones – the thumbnail is then cropped on the blog page.

Can anyone help me with adding code that will sort this? I just want it back to how it was before, I don’t want the images to be cropped.

http://www.epd.eu is the website.

Thanks, I know this isn’t very clear but happy to try and clarify.