Three people are posing on the side of a mountain.
The three people are posing at the mountain.
Posing at the mountain is the same as posing on the side of a mountain by the three people.
"Three people posing on the side of a mountain" is the same thing as "three people posing at a mountain".
"Three people are posing at the mountain" is a rephrasing of "Three people are posing on the side of a mountain."