We can make assumptions about what they are doing or feeling at the time of the photo - that is all. We certainly cannot tell what they are thinking.What do you mean "Yes, but"? A photo is a representation of the physical appearance of the person photographed. The physical appearance of the person as photographed may "indicate" what the person is thinking or feeling. Many photographs of people give very revealing indications of what the person is thinking or feeling.
But that is beside the point. My point is that just as a photo is a representation of the physical appearance of the person photographed (whether or not one has any indication of how the person is feeling), so modern brain scans (so I have been led to believe) can show areas of the brain firing up when certain activities are undertaken or emotions felt. To that extent (it is early days) these scans show something of what a person is thinking or feeling. To that extent the brain scanned person's consciousness is "visible".
We can take a sample of blood from someone and run tests that will give us a picture about the health of the individual at the time of the blood test.
We certainly cannot determine what they were thinking when the blood sample was taken.