It’s hard to find the right one and it’s buggy, but it’s there!
The real one from Prisma Labs will say the real website which is Prisma-ai.com so don’t let the rest of them fool you.
AND here for IOS
And you can’t just save photos on demand like in the iOS version. There’s no icon for that yet. You can email them to yourself, and then back up in the app to change the style, or you can go to settings and choose save them automatically. When I did this they didn’t seem to be saving, so I rebooted my phone (something you should do occasionally anyway) so it could start fresh and read all the configuration files. That’s when my gallery added a Prisma folder which had the previously saved pictures, now visible, and it collects the new ones, so I can post inside and outside the app.
And yes you can shoot outside the app and then modify with Prisma.
I saw a cropping issue before rebooting where it showed what I chose but trimmed extra off the bottom and left on the actual export, so watch for that.
P.S. When I tried the Share button to post to Tumblr it seemed to crash Tumblr when it opened and go back to Prisma. I discovered later that it opened Tumblr with the picture ready and then switched back. So if that happened to you, try tapping the app task bar and switching back to Tumblr so you can finish your post.
P.P.S. Not sure it posted everything I asked, but my Tumblr crossposts to Facebook are sometimes flaky, so not blaming the app yet.