lightbox2 - image classes

sorry to bother, searched for a while and am stuck on this one. i have installed drupal 6x and the image module along with lightbox. I have added a random image block and placed some pics to be displayed. Unfrtnately pics are opening up in a new page rather than through lightbox. I have checked out all settings and can only find in lightbox image handling tab I see possibly I need to add image classes for lightbox to work with.

I have looked for what to add and cannot find how i list image classes. Would it be something like *.gif ?

is there a different way to have all images on my site to open with lightbox

thanks for any support