Using a technique I call "reverse painting", you can use a paintbrush and water on a printed digital photo to manipulate the printed ink for different effects! In particular, you can turn a photo into a beautiful and unique painting.
Compare and contrast the two images shown (a "before" and "after") - make sure to click on the images to view larger versions and see the differences better.
The results shown were obtained with a digital photo print, water, a brush, and the techniques described in the next steps.
Step 1: Why and How It Works
Now, while the ink that the photo image consists of isn't necessarily water-soluble, the back-of-postage-stamp binding agent between the paper and the ink is.
This is why we can apply water selectively with a paintbrush to loosen, diffuse, and lift/wash away sections and layers of the printed photo. I call this technique "reverse painting" because instead of painting color onto the paper, you are instead selectively lifting it off and allowing it to remix or reflow (or be removed entirely). You control how this happens with your brush and water.
This process allows you to create beautiful and interesting effects, which I will cover in the next steps.
Note: In my tests, cheap photo paper ("Likon" 20-pack from the 1$ store) seems to work better then the more expensive kinds (such as HP Photo Print).
Step 2: Some Basic Techniques
The basic techniques are: The Eraser, The Smudge, and Sanding.
The images in this step demonstrate these basics. The next step covers applying to entire photos to make them look like paintings.
You can experiment to find your own techniques, too!
Step 3: Making Photo Prints Look Like Paintings
Applying more water and being careful to stay in color boundaries (or carefully crossing them), then allowing to dry can give make the photo look like a watercolor painting. I find that it's best to work with a photo that has bright colors, is high-contrast and high-brightness, and is not too busy or "dense" for this effect to be good.
Shown is a lucky cat figurine which has been made to look like a watercolor painting. Also included is the flower from the intro, shown halfway done.
Have fun and don't be afraid to try something new! Fiddle about, and discover your own techniques and share them in the comments, or better yet, make your own Instructable to showcase it!