Photos of three male teachers...
I put "der Mann". It was marked wrong and the feedback was that it should be "Lehrer". I suggest you add a photo of at least one female teacher, otherwise, both are correct.
In high school German, I was taught that "Lehrer" is a male teacher, and a female would be "Lehrerin" (most female versions of a noun had "-in" added to it). I usually read the hint above the pictures where it says what word you're translating, whether it's man or teacher.
OK. I've never tried the hint thing so I could try that in future but der Mann is still correct for a collection of three pictures of men (who happen to also be teachers). And if Lehrer is a male teacher and Lehrerin is a female teacher, then which word would I use for a set of photos that comprised 2 male and 2 female teachers?
Yes, der Mann is still correct it's just not what they wanted. If you're just talking generic teachers, they may decide to use Lehrer, just like in English we tend to say man or he when we actually are referring to anyone, male or female. I would assume that this website focuses on the generic form of teacher (Lehrer) if pictures of both are shown. Just look at the hint (it should appear above the pictures) and you'll get the answers.