When you have an existing photo of a wheel (e.g., previously taken by the Camera app that comes with every iOS device, previously imported into the Photos app that comes with every iOS device) you can choose it in the WheelSpotter app.
The best practice is when the photo is hi-resolution (i.e., greater than one megapixel).
The best practice is when the photo is square. (A round wheel composed in a square photo has less background compared to a rectangular photo.)
The Photos app that comes with every iOS device can be used to crop a rectangular photo into a square shape. In the Photos app, find the photo and tap the word EDIT, then tap the crop icon, then tap the icon to change aspect ratio and select square.
