The key thing is to have a photo. They, of course, must be owned by you but there are also many available that can be purchased via stock sites. Many of the strokes / dabs I have created on this site have either started out as photos or photo-realistic images (such as generated in Daz Studio or Poser).
The photo above is in color. This is OK but for a brush, a photo really needs to be in grayscale. Now you could just go to the desaturate command found in the image menu but a better command would be the image menu / adjustments / black and white as this allows for a great manipulation of the conversion of the photo into a dab. You can also combine it with other powerful filters such as HDR toning. Another option would be just to ignore that the image is in color and just define it as a stroke anyway. Using the black and white filter on the above image a number of variant black and white brushes can be created. You can also add all kinds of other effects into the mix as well to a create a range of variant tools / presets.
OK, now you have the black and white image, you can now define the photo as a brush. Personally, I dislike the edges of the photo so I would now suggest adding some additional white to the image. How to add white to an image ? Before you start, set the background color to white via the toolbox (toolbar). I would add a frame to the photo via either image menu and canvas size (add some additional size to the width and height). Another quicker option is to use the crop tool found in the toolbar (assuming it has not been moved by using the customization feature and if it has you will now find it in the extra tools section) and just increase the size of the entire photo by dragging the crop area wider and taller than the image. Now you have an edge around the photo. Of course, with this edge you still have a sharp edge for the definition of the photo brush - the next stage is to add a little frame / edging to the stroke. Set the foreground color via the toobar to white. Choose a standard stroke or an edgy brush stroke (or any of the existing presets) and apply the stroke around the edge of the Photoshop. Anything to slightly roughen or blur or smear the edge of the photo.
Now you have the required image (or something similar), go to the edit menu and select the define brush (or preset) command. The photo is now stored as a stroke in Adobe ® Photoshop ®.
Clearly, your strokes from photos will vary from the above. It depends on the original photo. Perhaps you will just want a single part of the image for the photo stroke ? In that case, simply use the rectangular marquee tool from the Photoshop toolbox and use the define brush command with that selection. The example below uses the eye of the original photo as the new brush stroke. The image was also modified using the new Photoshop iris blur filter. The eye was then selected and the edit menu define brush preset command used to store the photo to the preset panel. You could also carefully select the eye by using a circular marquee or using the other selection tools to highlight the eye. You could also apply the same approach as before and use the white color and the brush tool to remove out any area not required for the brush. You could also apply blur or feathering to the selection of the photo and then use the define command to save part of the photo as a stroke / dab.
I have created a number of Photo Letter Brushes. You can, of course, take a photo and manipulate it in 1000s of ways and save those to strokes. An example can be found below which was a set of photos taken of pages from a book. Instead of using a scanner, I used a camera to record the images and by bending and twisting and smudging and blurring (as below) the pages so many different types of dabs were created from the photos and they can all be used in Adobe ® Photoshop ®. You should check the copyright issues for any individual or building etc in the photo in your country
You can always use books etc as a great source for all kinds of dabs though if you want to actually use them as a set of strokes, probably best to find a very old Public domain book (and I would suggest checking with your local laws on what is out of copyright etc). The text below comes from a book from about 1740 and I doubt if the writer is around now (I guess)