It's an error message that should signal that in the provided photo there is more than 1 person and the user should upload a photo where they are alone. Can I use in this context the word "solo" to describe the kind of photo we need? Or does it mean that the photo was done by the user without anyone's help? Is there any other word/phrase that describes the kind of photo where you're alone?
Sentence: Upload a solo photo of you. VS Upload a headshot photo of you alone.
Does the "solo photo" sound weird in this context?