resizing to display a large image

I've tried searching but couldn't find this issue, apologies if its a duplicate.

in my node I have thumbnails and then clicking on the thumbnail will show the full size image in
I'm using colorbox to display the larger image and it works great :)

however Some of the images are larger than the browser and when these are opened by colorbox, it resizes them down to fit the browser window. I don't have a problem with this, but is there a way for the user to resize the image to its actual size?