The reason is on the item itself, naming which image it was. Replace that one and it goes back into review automatically.
The reason is shown against the image itself and names which one — a listing can have both a cover image and a paid file, and both are checked.
What it means
Either the automatic scan flagged it, or a person reviewed it and it does not meet the content rules.
Fixing it
Replace that one image. It goes back into review automatically and nothing else about the item is affected.
If you think it is wrong
The automatic scan is tuned to avoid false positives — swimwear, gym photos and combat sports pass. It is not perfect. Contact support with the item and a person will look; nothing is deleted for being flagged, so there is nothing to recover.
Why we do not quote the exact label
The scan returns a probabilistic guess, and repeating a raw machine label reads as an accusation when it is wrong. What you get instead is the category and what to change.