image post enlarge on click

Hi
I am using 2.9.2

the site is running on theme: grid-a-licious

i can insert an image into each post with “Post Image” plug-in

however, there seems to be a limit to my post image sizes as they appear on the main page – even if the image is larger than 600 px wide

http://facedesign.com/log/

how can i insert a thumbnail into a post and then once i click it shows the full size image (in a different page or on top of the screen like a lightbox)

is there a setting i need to change?

thank you