You generally cannot tell for sure from a picture alone
The only reliable way is direct confirmation from the person or a medical professional
In flaccid photos, the foreskin may cover the glans if uncircumcised
In circumcised photos, the glans is usually fully exposed
Some uncircumcised people may have a retracted foreskin, making it look circumcised
Some circumcised people may have a bit of remaining foreskin or skin that changes appearance
Lighting, angle, erection state, and image quality can make it impossible to tell
If you need to know for personal or medical reasons, ask respectfully and directly