Selection of Pictures

Use a Multiple Choice (Radio Button) field. In the choices you can use HTML. Place HTML to display an image for each radio button. Enable values for the choices and for the value for each choice give it a friendly (non-HTML) name so you know which picture it is.

For example, see this screenshot of the choice settings: http://grab.by/aqbl

You would have to place your images on your server and in the SRC put the full HTTP path to the image. So you need to understand HTML in order to implement this properly.

There is currently no integration with the NextGen Gallery. Because NextGen is not a commercial product with a longterm development plan it's difficult for us to partner or integrate with them because there is nobody official for us to work with to ensure future compatibility.

The only way to insert images into the choices in the field is by inserting HTML into the choices. There is currently no integration with the WordPress gallery.

The choice name field accepts HTML, so if you want a hover state or some sort of link that displays a bigger version then you'd have to include that in the HTML.

Grabriel,
What I would suggest is for you to find a script online that has the functionality you need for the images and implement it with a simple HTML page. Once you get it working, try integrating that HTML page with Gravity Forms. As Carl already explained, you can add HTML to the radio button's choice field, and you can always add HTML fields if you need to add any extra HTML code to the form.