One of the most obvious ways to work a photo into an art project is by using the majority of your project as a frame. Include some drawings which you feel connect to the theme of the photo, and use colors which compliment or contrast with the colors of the photo. Compare the color of the photo's borders to a color wheel. If a color is next to another color on the color wheel, it is said to compliment, if it is opposite, it contrasts.
Not every photo is perfect, but on some pictures you can make a less ideal photo look better by what is called cropping. Cropping is cutting parts of the photo off to remove the excess background and make it a more streamlined-looking photo. This is often done on computers, but you can also use a pair of scissors for the same effect. A smaller photo can have a striking impact on a mixed media project. You could even remove all of the background and use the image as a sticker.
Like a mask, a photo can cover (or be covered by) parts of your project. You could use this to do something like add a beard, clothing, or other accents to an existing photo, or add someone's face or images of real-world objects to your drawing. You can use photos which have been cropped, or larger photos drawn into your creation. This is called masking because it covers part of your drawing with a non-transparent photo.
When you take a photo of a landscape, for example, there are things which the lens cannot capture. The camera can only see up to the edges of the camera lens and no further. Because you are an artist, you have creative license to add to the outside of the photo. You could extend the lines of your photo to add to the image around its edges. This could allow you to make a new cityscape or make up your own details that the camera "missed."