If it’s a photo of screen/screenshot and you have clear reason to think it is not that person’s pic, it’s reasonable to flag as copyright infringement, since, misattributed photos also get flagged for this.
Some evidence I have seen that these types of photos aren’t attributed correctly include that they’re posts on FB or instagram, other people’s usernames are visible, the photo is clearly from a test or other assessment, etc. I’m sure there are more!
It’s also generally best to leave a polite comment explaining the issue to the user. Many newer users will not know that uploading screenshots/pics of screens can be an issue and can then improve in the future. If it is a repeated pattern, like someone uploading a series of screenshots for a project or something to try to get credit/meet a required number of observations, I generally comment on the first few, and then flag the rest.
However, especially when it’s an isolated issue, we’re supposed to assume that other users mean well, and if it isn’t clearly a violation, it’s best to ask in a comment before flagging. I have seen some users who appear to be less tech savvy consistently uploading pics of their own screens and photos that they took. This isn’t ideal, but also not copyright violation.