If you want to add a photo of your meal for your own record keeping purposes, you can do so by simply tapping the More Options (⋮) button next to the name of the meal (Breakfast/Lunch/Dinner etc).
Then, select Add Photo.
You'll then be prompted to select a file. Tap Choose File.
Next, you can choose to take a photo, select a photo from your photo library, or browser the file structure on your device for a photo file.
Select whichever option is appropriate for your situation, and proceed with uploading the photo. The photo will then be displayed in the "food details" screen for the given log entry.