Number of bytes in the image file
We should stop making up new scoring schemes for programming languages, which only serves to make things more difficult. Code golf contests are scored in bytes, so that's what matters. Everything else is artifical and prone to not take all details into account.
For example, scoring Piet submissions just by the number of codels doesn't take the shape into account; it considers 4x1, 2x2, and 1x4 images with the same codels the same program, which obviously isn't true.
Number of codels in the image
All codels in the image will be counted. This includes codels that are never visited, and black and white codels.