How Sexy Can Photos Be That Are Sent to an Inmate?

The line is nudity. Anything that crosses into exposed private areas will be rejected by the mail room. Everything short of that is generally acceptable.

Bikini photos, lingerie, and suggestive poses are fine as long as nothing is explicitly exposed. Facilities inspect all incoming mail and photos, and images containing nudity will be confiscated before they ever reach your boyfriend. In some cases a facility may flag the sender's account for future scrutiny if explicit content is found, so it is not worth pushing the boundary.

When in doubt, think swimsuit or underwear coverage rather than less. That keeps the photos meaningful without risking rejection at the mail room.