Gallery image size

Hi, apologies if this has already been covered — I looked through several pages of queries but didn’t find an answer.

Q: After I add images to a post and I click on the main slider image to view the gallery, I see the thumbnails for the images (great!), but under the thumbnails I see a vertical queue of large versions of the same images. How do I get rid of these leaving only the thumbnails?

Thanks in advance for help.