Facility mail policies change, and nudity in photos is one of the most inconsistently enforced areas across correctional systems. What gets through one month can get rejected the next based on new guidance from administration, a change in mailroom staff, or a facility-wide policy update that was never publicly announced.
Sheridan Correctional Center is an Illinois Department of Corrections facility, and IDOC has a general policy that restricts sexually explicit material including nudity. The fact that some photos got through previously likely reflects inconsistency in how the policy was being applied at the mailroom level rather than an official permission to send nude photos. Individual mailroom officers use judgment in borderline cases, and that judgment varies from person to person and shifts over time.
The most direct way to find out what changed is to call the facility and ask to speak with the mailroom supervisor or a counselor. Ask specifically what the current policy is regarding photos, what types of content are permitted, and whether there has been a recent policy update. Getting that information in writing if possible gives you something to reference going forward.
The practical reality is that nudity has always been a gray area that facilities can reject at their discretion, and the safest approach going forward is tasteful but clearly intimate photos rather than explicit ones. Photos that stay on the right side of that line consistently get through. Photos that rely on a lenient mailroom officer to make the call will always be inconsistent.
The photos that already made it through are staying where they are. That much is certain.