Describe the place, talk about the people and what they might be doing.
You can start like this:
In the last picture I can see a woman. She could be a manager of a big multinational company. She must be working on a serious project because…
In your description use modal verbs of speculation:
The language structures you can use:
She may be working.
She must be at work.
Peter can´t be in London. He is in Milan.
They might be working or studying.
The café may offer faster Wi-Fi than at home.
They could be travellers needing internet access.
The network must be popular if everyone is using it.
Some customers might not feel safe using public Wi-Fi.
The phone is ringing. It could be John / It may be Tom. / It might be Tom.
For the past we use:
He must have been ill.
Jane can´t have gone out.
They may have visited Prague.
They might have seen the project.