Q: I'm very interested in using this app as an alternative to selling digital images. I know that I ultimately control the image resolution, watermarks, etc. But regardless of all that, I still want to know: How easy is it for someone to download the pictures to get them out of the app?
A: Easy... a screen capture is simple to do on a mobile device. This is the nature of putting images out on the internet... if people can see them, they can grab them. The app has the ability to let people download as well, but that can be disabled with a setting on the gallery. In the Pro subscription you can also disable right click on a computer but at the end of the day, people will still be able to use a variety of methods to screenshot the images when they are displayed and many are happy with the low-resolution image this produces.
Like you mentioned, the best way to protect your images is to watermark them or control their size.
Here is a link to an article that discusses image protection in general.
And one on how to disable right click on a computer.