Why does some images not work as thumbnails for lists?

Does anyone know why some images won't work as thumbnails for lists? I know it's a minor issue, but it's still a little annoying when the number 1 item on your lists isn't whats being shown as the thumbnail. Especially in lists like this (www.listal.com/list/my-favorite-films-henrik4075) where i prefer the top four movies to be what's shown in the preview. Is there a specific reason some images won't work or is it just random?

Got the same Problem at new Lists..Not the first - a random Pic of my List was show as the " Main Thumb " for the List.Try to rearrange it,but it failed.
www.listal.com/list/sexy-sweater-babes-part-5

I received this explanation from Tom several years ago:

I can no longer choose which image appears as the thumbnail on image lists.

www.listal.com/list/artwork-lucio-parrillo-mutoscope

"It's because all the images in the list have been classified as NSFW and the list is not marked as NSFW. I've made it so for a non NSFW list you must select a SFW image for the thumbnail for it."

My lists are just for regular movies, not girls in swimsuits or whatever and i still have this problem. This is my most recent list ( www.listal.com/list/best-michelle-pfeiffer ) and none of the images are NSFW and they still won't show as the thumbnail. It just shows the movie posters.

Luckily, Henrik, yours appears to be fixed. Both banner and thumbnail show properly. My last two lists, however, are still broken. Not sure what this issue it.

Edit: I take it back, thumbnails appear to be working for my list again. I just published one to see if that would help, apparently it did.

seems to work, my first Pic is showing up as Main Image now