I've created a view for a group of nodes, what I want to do is output the image just for the first result rather than all of them.
Currently I'm using CSS to hide all except the first but that isn't ideal.
Display Suite provides this functionality. Just install DS, then, in your view, choose "Display Suite" under the option for "Show":
In the settings for that option you can choose a different view mode for the first item in your list: