I know the reason...

I upload pictures to picasa and picasa make out different sizes thumbs acorrding the original pic,but in fact,I have only 1 picture-the original one in my picasaweb,so when I click on the thumb,the larger one can't show,because they two are the one,just have different size.If I want to use lightbox,I have to upload 2 pics,one is thumb,the other is large pic,and be sure they're two.So this means I can't use lightbox,because my picasa have the limited size-1GB,I can't upload 2 kinds of pics everytime,this is a waste.