How to change the size of the images shown on galleries

Hi,

I upload multiple images using imageFUpload.
I then categorise them under an image gallery term.
When I visit my gallery I see the little thumbnails of my images.
When I click on an image from a gallery I see them in a Preview size whereas I want to show them in their original size.

Is there any way of doing this? I have spent lots of days trying to find this but haven't been lucky so far.

Thx