My Co worker sent me this, she's not wearing a bra...

Don’t mention the bra—especially since she’s a coworker. Compliment the dress and how confident she looks.

A good reply:

“You look amazing in it. The dress is elegant, really flattering, and the white looks great on you.”

A slightly more flirtatious version, only if that tone is already mutual:

“Honestly, you look incredible. That dress really suits you.”

Keep it warm, but avoid comments about her chest or what she is—or isn’t—wearing underneath.

That’s an intimate photo, so treat it as private and don’t share or save it without her permission. It suggests flirtation, but it still doesn’t automatically mean consent to anything beyond receiving the photo—especially because she’s a coworker.

A respectful reply could be:

“You look beautiful. Thank you for trusting me with something this private—are you comfortable with me telling you what I’m thinking?”

That shows interest while letting her set the tone. Avoid explicit comments about her body unless she clearly invites them.