When you upload a photo to a post in a thread, click on Preview and you'll see the photo there. Well, in the post itself is a special tag that starts with '[lightbox]'. That tag is what is producing the image you uploaded.
So if you upload two images, you'll have two lightbox tags and when you preview the post, the images will show in the order that the corresponding tags are entered. So you can cut and paste to move the top lightbox tag below the other one and that'll change the order of the images. Just click on Preview after you make your changes to see. Be careful not to change the lightbox code as that'll break the image.
