If you are talking about image captions on single pages e.g. http://themes.designorbital.com/contango/portfolio/ornare/
You can add captions and image is automatically linked to big sized image to show in the light box.
Does the portfolio template always use only the Featured Image for both thumbnail and full size?
Contango uses the following image dimensions for various posts.
== Minimum Recommended Image Dimensions: Width x Height ==
Blog Posts: 580 x 350
Portfolio Posts: 940 x 600
Slide Posts: 940 x 450
Client Posts: 340 x 150
Portfolio template will try to get the image automatically with a pre-defined sequence from Featured Image to Images attached to the Post.