Scanning will get you the best images if you have access to one. Remove white-space around the image by cropping.
You can also get a pretty decent photo for your submission using your smartphone. If you’re using an iPhone you can enable a grid view to your camera which will help you get your image a little straighter. To do this, go to your phone’s settings -> Camera -> flip the toggle next to grid. You can also crop images directly from your phone with the ‘Edit’ function. There are also several free apps you can download to help you take high quality images on your phone for your submissions.