First, you just need to open the app as you would normally, then click the blue Upload a picture button, indicated below.
Once that's done, you'll see the option to either choose an image from your gallery or take a photo with your camera.
Choose (or take) your photo. You can write a comment just below the photo and once that's done just click the green Upload button to send it direct to the owner's inbox. It's that simple!
Please bear in mind you can't send pictures while a walk is being tracked, so if you're doing both you should take pictures as normal on your phone then send them via your gallery once the walk is complete.