Sometimes there are faulty pictures. Entirely black or entirely white.
I have been marking them as "no penguins so far, but I was suddenly wondering if I should mark them as "can't tell".
Depends on what that tag is used for.
Is it to decide how often the picture is shown to volunteers or which pictures need a pro to look at, then "no" makes most sense.
Or is it used in the database to mark that on dateX, on time Y there were no pinguins on locations "Z".
Becasue then "can't tell" would be a better marking.
I'm just a moderator, but as I understand to the goals of this project, you should mark these 'wrong' (dark, blurry.. etc) images as 'I can't tell', because there can be some penguins in there, they're just not visible on the screen.