[support] Lightbox2 and image field

Hi all, I want to use Lightbox2 on an image field in a content type. I have done this before, but for the life of me can't get it running again. :-( I'm using D7 and Lightbox2. I have added an image field to a content type. In the site I configured before I could set the Format of the image to use Lightbox2 with a specific size. Now I can't get this formatter activated. Anybody know how to get this running? Thanks Bert