The other day I had the task to display the image of a multi-upload image field with a bigger image style if there was only one image uploaded.
I solved it by using a combination of the following modules:
- Field formatter conditions
- Field formatter settings (required by Field formatter conditions)
- Computed Field
In the display settings of my content type I created a duplicate of my image gallery field using Display Suite's Dynamic Field. (You could also achieve this by using Display Suite's Code Field or creating a Computed Field that just outputs the value of your gallery field.)
For that new instance of my gallery field I set the desired bigger image style and the instruction to only show the first image if there are multiple.
Now I had the small gallery thumbnails and the big image being shown on node view.