You cannot find it online. Federal Bureau of Prisons facilities do not make inmate booking photos publicly available. The BOP inmate locator provides basic information like name, register number, release date, and facility, but photos are not part of the public record for federal inmates.
Mugshots that appear on third-party websites almost always come from county jail bookings, which are public record in most states, or from state DOC systems that make photos part of their public offender database. Federal inmates are not included in either of those sources.
If you need a photo for a legitimate legal or administrative purpose, your attorney may be able to request one through formal channels, but there is no public-facing way to retrieve a federal inmate booking photo.