Take ones similar to the ones you posted, but with good lighting instead of a flash, and not in a mirror. The ruler / tape measure suggestion is good, but it's very difficult to do. It's hard to operate the camera, even with a remote, while keeping your penis and ruler in the right position and still maintaining an erection with the distractions. It's also hard to make the numbers on the ruler show up. You can do it, but unless you have help, it will probably take several tries.