What is PNG?
PNG is the web's lossless image format: compression that never degrades the picture, and a real alpha channel for transparency. It is the right choice for screenshots, logos, line art and anything with sharp edges or text.
What is BMP?
BMP is the Windows bitmap: pixel values written out with essentially no compression, which makes the files enormous and the format trivial to read. It survives in Windows tooling, embedded systems and anywhere a program wants image data without a decoder in front of it. Rows are stored bottom-up.