"We are seeing" (present continuing) is not generally used in English to mean sehen except in specific circumstances. That seems to be Duolingo's policy, anyway, as it's one situation where it clearly prefers using simple present tense instead ("we see").
That's not a good translation. "Various" has an emphasis on there being some difference between each of the photos, which manche does not contain. "Some" is a good translation that does not change the meaning or emphasis.
Auf is a 2-way preposition. When describing motion, like putting something ONTO the table, you use the accusative den. When describing a static location, where the photos happen to BE, you use the dative dem.