Image sizes: preventing them from changing

Hey,

Im using the Core theme ( http://themeforest.net/item/core-minimalist-photography-portfolio/240185 ) for my photography website ( http://www.danielspellman.com ).

My problem is that I carefully resize and optimise images for the web in Photoshop, then they get resized very badly when viewed on my site. Small images become bigger and bigger images become smaller. The support for the theme havnt helped me so I thought Id ask here…

How do I have the lightbox image display images at their actual size? I was told they are being resized to match the user’s screen size/res. I dont want that to happen.

Thanks so much!