Ok, here was something that wasn’t immediately explained to me so Google saved the day and now I understand. A power of two image is an image where either side of the image is a whole number when you find the square root. So, 1, 2, 4, 16, etc. are ok because they can be square rooted. However, 5, 7, 511 are not good because they cannot be square rooted to a whole number.This website helped me understand:
A number with a square root isn’t necessarily a power of two. For example 25 has an square root, 5, but is not a power of two.
A power of two is a number whose Log base 2 is a whole number. Or by definition 2^n where n is any integer > 0.