I was wondering if anyone knew how to do this. There are lots of apps for password protecting your pictures, but you can't export or share the picture through text message once it is password protected, there is no option for this.
I use this app It allows me to import the pictures into the protected folder and keeps the pictures on the phone (so no external deals). Unfortunately it doesn't have the feature to share them directly, but you can always export the picture(s) you want back to saved photos and share them from there.
Also you could mail the dev and ask him if he could consider adding the feature you want. I'm recommending this particular app, because the dev has some very good apps