[Plugin: Lightbox Gallery] Incorrect number of pictures?

I have a strange problem with Lightbox-gallery. I created a gallery with 12 pictures. It shows 12 thumbnails, but when I click on a thumbnail, under the zoomed-in version it says “Image x of 13”. But the number of images is definitely 12. I noticed that when I navigate through images, the first image is displayed twice, and the second one doesn’t have any info in the caption.

So I did an experiment and removed the first image from the gallery. Now there are 11 images, but it says there are 12 and again the first image is displayed twice.

Any ideas?

P.