Improve internal tests when using real images input

[schmidt9]

Reference: #10
When testing real images from assets I was unable to achieve correct difference images comparison inside assertComparison method (commit dbdd914)

Unable to get equality between difference images #10 (comment)

I suppose (but not sure) issue may be related to some different PNG parameters of asset images and images generated with method.compareImages. Maybe before difference images comparison we should normalize them by converting to grayscale since they are both white and black, but can have different color spaces (RGB vs Grayscale)