Where do I find a picture I sent from messages app as MMS? I took a picture of a tracking number receipt and sent the text message. Now I can't find the picture of the receipt. How do I find it? It's not in gallery.
If you're using the Google messages app, then the picture wouldn't automatically be saved to your gallery app, rather, it stays in the messaging app. 

Open the messaging app, find the person you sent the message to, and the pictures should be there. If you deleted the picture, then it's fully deleted and the only way to recover it, would be by using some picture recovery method.

If your phone is rooted, you can double check in the directory where all SMS/MMS are stored, it might still be there. This would be under /data/data/com.android.providers.telephony/databases/mmssms.db